Introduction to Perl

    123  12.4  5E-10  0xff (hex)  0377 (octal)

    'What you $see is (almost) what \n you get'   'Don\'t Walk'

    "How are you?"  "Substitute values of $x and \n in \" quotes."

    `date`   `uptime -u`   `du -sk $filespec | sort -n`

    $x    $list_of_things[5]     $lookup{'key'}

Single-quotes ' ' allow no substitution except for \\ and \'. Double-quotes " " allow substitution of variables like $x and control codes like \n (newline). Back-quotes ` ` also allow substitution, then try to execute the result as a system command, returning as the final value whatever the system command outputs.

    ('Sunday', 'Monday', 'Tuesday', 'Wednesday',
        'Thursday', 'Friday', 'Saturday')

    (13,14,15,16,17,18,19)   equivalent to  (13..19)

    (13,14,15,16,17,18,19)[2..4]  equivalent to  (15,16,17)

    @whole_list

    $DaysInMonth{'January'} = 31;   $enrolled{'Joe College'} = 1;

    $StudentName{654321} = 'Joe College';  

    $score{$studentno,$examno} = 89; 
	
    %whole_hash

Perl 5 allows combinations of these, such as lists of lists and associative arrays of lists.

Name Conventions

Scalar variables start with '$', even when referring to an array element. The variable name reference for a whole list starts with '@', and the variable name reference for a whole associative array starts with '%'.

Lists are indexed with square brackets enclosing a number, normally starting with [0]. In Perl 5, negative subscripts count from the end. Thus, $things[5] is the 6th element of array @things, and

    ('Sun','Mon','Tue','Wed','Thu','Fri','Sat')[1]

Associative arrays are indexed with curly brackets enclosing a string. $whatever, @whatever, and %whatever are three different variables.

    @days = (31,28,31,30,31,30,31,31,30,31,30,31);
                   # A list with 12 elements.

    $#days         # Last index of @days; 11 for above list

    $#days = 7;    # shortens or lengthens list @days to 8 elements

    @days          # ($days[0], $days[1],... )

    @days[3,4,5]   # = (30,31,30)

    @days{'a','c'} # same as ($days{'a'},$days{'c'})

    %days          # (key1, value1, key2, value2, ...)

Case is significant--"$FOO", "$Foo" and "$foo" are all different variables. If a letter or underscore is the first character after the $, @, or %, the rest of the name may also contain digits and underscores. If this character is a digit, the rest must be digits. Perl has several dozen special variables whose second character is non-alphanumeric. For example, $/ is the input record separator, newline "\n" by default. An uninitialized variable has a special "undefined" value which can be detected by the function defined(). Undefined values convert depending on context to 0, null, or false.

The variable "$_" Perl presumes when needed variables are not specified. Thus:

    <STDIN>;          assigns a record from filehandle STDIN to $_
    print;            prints the curent value of $_
    chop;             removes the last character from $_
    @things = split;  parses $_ into white-space delimited
                      words, which become successive
                      elements of list @things.

Subroutines and functions are referenced with an initial '&', which is optional if reference is obviously a subroutine or function such as following the sub, do, and sort directives:

    sub square { return $_[0] ** 2; }

    print "5 squared is ", &square(5);

Filehandles don't start with a special character, and so as to not conflict with reserved words are most reliably specified as uppercase names: INPUT, OUTPUT, STDIN, STDOUT, STDERR, etc.

8. Literals and Operators

Example: Numbers and Characters

    #!/usr/local/bin/perl
    print '007',' has been portrayed by at least ', 004, ' actors. ';
    print 7+3, ' ', 7*3, ' ', 7/3, ' ', 7%3, ' ', 7**3, ' ';
    $x = 7;
    print $x;
    print '   Doesn\'t resolve variables like $x and backslashes \n. ';
    print "Does resolve $x and backslash\n";
    $y = "A line containing $x and ending with line feed.\n";
    print $y;
    $y = "Con" . "cat" . "enation!\n";
    print $y;

This produces:

    007 has been portrayed by at least 4 actors. 10 21
    2.3333333333333335 1 343 7
    Doesn't resolve variables like $x and backslashes \n. Does
    resolve 7 and backslash
    A line containing 7 and ending with line feed.
    Concatenation!

Questions:

1. Why does the output from the first few print statements run together?
2. Is it necessary to declare variables in Perl? Is it possible?

Example: Comparisons

    # The following "<<" variation of
    # data input simplifies CGI forms.
    $x = 'operator';
    print <<THATSALL;
    A common mistake:  Confusing the assignment $x =
    and the numeric comparison $x ==, and the character
    comparison $x eq.
    THATSALL
    $x = 7;
    if ($x == 7) { print "x is $x\n"; }
    if ($x = 5)  {
        print "x is now $x,",
        "the assignment is successful.\n";
    }
    $x = 'stuff';
    if ($x eq 'stuff') {
        print "Use eq, ne, lt, gt, etc for strings.\n";
    }

This produces:

    A common mistake:  Confusing the assignment operator =
    and the numeric comparison operator ==, and the character
    comparison operator eq.
    x is 7
    x is now 5, the assignment is successful.
    Use eq, ne, lt, gt, etc for strings.

Example: Ordinary Arrays

    @stuff = ('This', 'is', 'a', 'list.');
    print "Lists and strings are indexed from 0.\n";
    print "So \$stuff[1] = $stuff[1], ",
         "and \$#stuff = $#stuff.\n";
    print @stuff,"\n";
    print join('...',@stuff),"\n";
    splice(@stuff, 3, 0, ('fine', 'little'));
    print join('...',@stuff),"\n";

This produces:

    Lists and strings are indexed from 0.
    So $stuff[1] = is, and $#stuff = 3.
    Thisisalist.
    This...is...a...list.
    This...is...a...fine...little...list.

Homework: Validate a date.

    #!/usr/local/bin/perl
    print "Enter numeric:  month  day  year\n";
    $_ = <STDIN>;
    ($month,$day,$year) = split;

Approach this incrementally. On the first draft, assume that the user enters 3 numbers separated by spaces and that February has 28 days. Subsequent refinements should account for bad input and leap year. Finally, find a Perl builtin function that converts a date to system time, and see how to use that to validate time data generally.

Homework: Play with associative arrays.

Start with a few assignments like:

    $name{12345} = 'John Doe';
    $name{24680} = 'Jane Smith';

    $blunk = %name;
    @persons = %name;
    print '$blunk=',$blunk,', @persons=',
        join(', ',@persons),"\n";

Example: Command Line Values and Iterative Loops

    print "$#ARGV is the subscript of the ",
        "last command argument.\n";
    # Iterate on numeric subscript 0 to $#ARGV:
    for ($i=0; $i <= $#ARGV; $i++) {
        print "Argument $i is $ARGV[$i].\n";
    }
    # A variation on the preceding loop:
    foreach $item (@ARGV) {
        print "The word is:  $item.\n";
    }
    # A similar variation, using the
    # "Default Scalar Variable" $_ :
    foreach (@ARGV) {
        print "Say:  $_.\n";
    }

    > perl example5.pl Gooood morning, Columbia!
    2 is the subscript of the last command argument.
    Argument 0 is Gooood.
    Argument 1 is morning,.
    Argument 2 is Columbia!.
    The word is:  Gooood.
    The word is:  morning,.
    The word is:  Columbia!.
    Say:  Gooood.
    Say:  morning,.
    Say:  Columbia!.

Example: Standard I/O

    print STDOUT "Tell me something: ";
    while ($input = <STDIN>) {
            print STDOUT "You said, quote: $input endquote\n";
            chop $input;
            print STDOUT "Without the newline:  $input endquote\n";
            if ($input eq '') { print STDERR "Null input!\n"; }
            print STDOUT "Tell me more, or ^D to end:\n";
    }
    print STDOUT "That's all!\n";

Note 2: Data records are by default terminated by a newline character "\n" which in the above example is included as the last character of variable $input. The "chop" function removes the last character of its argument. Perl 5 introduces a "chomp" function that removes the last characters of a variable only if they are the currently defined end-of-record sequence, which is defined in the special variable $/.

Demonstration:

    > perl example6.pl
    Tell me something: I'm warm.
    You said, quote: I'm warm.
     endquote
    Without the newline:  I'm warm. endquote
    Tell me more, or ^D to end:
    Can I have some water?
    You said, quote: Can I have some water?
     endquote
    Without the newline:  Can I have some water? endquote
    Tell me more, or ^D to end:

    You said, quote:
     endquote
    Without the newline:   endquote
    Null input!
    Tell me more, or ^D to end:
    ^D
    That's all!

Example: Perls, A Perl Shell, Calculator, & Learning Tool

    #!/usr/local/bin/perl
    for (;;) {
	print '(',join(', ',@ReSuLt),') ?';
	last unless $InPuT = <STDIN>;
	$? = ''; $@ = ''; $! = ''; 
	@ReSuLt = eval $InPuT;
	if ($?) { print 'status=',$?,' ' }
	if ($@) { print 'errmsg=',$@,' ' }
	if ($!) { print 'errno=',$!+0,': ',$!,' ' }
    }

    $InPuT = <STDIN>;               # Get line from standard input.
    if (! defined($InPuT)) {last;}  # If no line, leave the loop.

Demonstration:

    perls
    () ?Howdy
    (Howdy) ?2+5
    (7) ?sqrt(2)
    (1.4142135623730951) ?$x
    () ?$x = sqrt(2)
    (1.4142135623730951) ?$x + 5
    (6.4142135623730949) ?1/0
    errmsg=Illegal division by constant zero in file (eval) at line 2, next 2 tokens "0;"
    () ?system 'date'
    Fri Sep 27 10:02:43 CDT 1996
    (0) ?$x = `date`
    (Fri Sep 27 10:02:52 CDT 1996
    ) ?chop $x
    (
    ) ?$x
    (Fri Sep 27 10:02:52 CDT 1996) ?@y = split(/ /,$x)
    (Fri, Sep, 27, 10:02:52, CDT, 1996) ?@y[1,2,5]
    (Sep, 27, 1996) ?localtime()
    (37, 4, 10, 27, 8, 96, 5, 270, 1) ?
    () ?foreach (1..10) {print sqrt(),' '}
    1 1.4142135623730951 1.7320508075688772 2 2.2360679774997898 
    2.4494897427831779 2.6457513110645907 2.8284271247461903 3 3.1622776601683795    
    () ?exit

Example: File I/O

    #!/usr/local/bin/perl
    # Function:  Reverse each line of a file

    # 1: Get command line values:
    if ($#ARGV !=1) {
        die "Usage:  $0 inputfile outputfile\n";
    }
    ($infile,$outfile) = @ARGV;
    if (! -r $infile) {
        die "Can't read input $infile\n";
    }
    if (! -f $infile) {
        die "Input $infile is not a plain file\n";
    }

    # 2: Validate files
    # Or statements "||" short-circuit, so that if an early part
    # evaluates as true, Perl doesn't bother to evaluate the rest.
    # Here, if the file opens successfully, we don't abort:
    open(INPUT,"<$infile") ||
        die "Can't input $infile $!";
    if ( -e $outfile) {
        print STDERR "Output file $outfile exists!\n";
        until ($ans eq 'r' || $ans eq 'a' || $ans eq 'e' ) {
            print STDERR "replace, append, or exit? ";
            $ans = getc(STDIN);
        }
        if ($ans eq 'e') {exit}
    }
    if ($ans eq 'a') {$mode='>>'}
    else {$mode='>'}
    open(OUTPUT,"$mode$outfile") ||
        die "Can't output $outfile $!";

    # 3: Read input, reverse each line, output it.
    while (<INPUT>) {
        chop $_;
        $_ = reverse $_;
        print OUTPUT $_,"\n";
    }

    # 4: Done!
    close INPUT,OUTPUT;
    exit;

This example produces a score summary report by combining data from a simple file of student info and a file of their scores.

Input file "stufile" is delimited with colons. Fields are Student ID, Name, Year:

    123456:Washington,George:SR
    246802:Lincoln,Abraham "Abe":SO
    357913:Jefferson,Thomas:JR
    212121:Roosevelt,Theodore "Teddy":SO

Input file "scorefile" is delimited with blanks. Fields are Student ID, Exam number, Score on exam. Note that Abe is missing exam 2:

    123456 1 98
    212121 1 86
    246802 1 89
    357913 1 90
    123456 2 96
    212121 2 88
    357913 2 92
    123456 3 97
    212121 3 96
    246802 3 95
    357913 3 94

The desired report:

    Stu-ID Name...                       1   2   3   Totals:

    357913 Jefferson,Thomas             90  92  94       276
    246802 Lincoln,Abraham "Abe"        89      95       184
    212121 Roosevelt,Theodore "Teddy"   86  88  96       270
    123456 Washington,George            98  96  97       291

                              Totals:  363 276 382

The program that made this report:

    #!/usr/local/bin/perl
    # Gradebook - demonstrates I/O, associative
    # arrays, sorting, and report formatting.
    # This accommodates any number of exams and students
    # and missing data.  Input files are:
    $stufile='stufile';
    $scorefile='scorefile';

    # If file opens successfully, this evaluates as "true", and Perl
    # does not evaluate rest of the "or" "||"
    open (NAMES,"<$stufile")
        || die "Can't open $stufile $!";
    open (SCORES,"<$scorefile")
        || die "Can't open $scorefile $!";

    # Build an associative array of student info
    # keyed by student number

    while (<NAMES>) {
            ($stuid,$name,$year) = split(':',$_);
            $name{$stuid}=$name;
            if (length($name)>$maxnamelength) {
                    $maxnamelength=length($name);
            }
    }
    close NAMES;

    # Build a table from the test scores:

    while (<SCORES>) {
       ($stuid,$examno,$score) = split;
       $score{$stuid,$examno} = $score;
       if ($examno > $maxexamno) {
          $maxexamno = $examno;
       }
    }
    close SCORES;

    # Print the report from accumulated data!

    printf "%6s %-${maxnamelength}s ",
            'Stu-ID','Name...';
    foreach $examno (1..$maxexamno) {
            printf "%4d",$examno;
    }
    printf "%10s\n\n",'Totals:';

    # Subroutine "byname" is used to sort the %name array.
    # The "sort" function gives variables $a and $b to
    # subroutines it calls.
    # "x cmp y" function returns -1 if x<y, 0 if x=y,
    # +1 if x>y.  See the Perl documentation for details.

    sub byname { $name{$a} cmp $name{$b} }

    # Order student IDs so the names appear alphabetically:
    foreach $stuid ( sort byname keys(%name) ) {
       # Print scores for a student, and a total:
       printf "%6d %-${maxnamelength}s ",
               $stuid,$name{$stuid};
       $total = 0;
          foreach $examno (1..$maxexamno) {
             printf "%4s",$score{$stuid,$examno};
             $total += $score{$stuid,$examno};
             $examtot{$examno} += $score{$stuid,$examno};
          }
          printf "%10d\n",$total;
    }

    printf "\n%6s %${maxnamelength}s ",'',"Totals: ";
    foreach $examno (1..$maxexamno) {
       printf "%4d",$examtot{$examno};
    }
    print "\n";
    exit(0);

Perl allows an associative array to be "tied" to a genuine database, such that expressions like $record = $student{$key} use the database. See the "dbm" and "tie" functions.

11. Pipe I/O and System Calls

    #!/usr/local/bin/perl
    # Report on disk usage under specified files
    # The Unix command "du -sk ..." (on BSD Unix, "du -s ...")
    # produces a series of lines:
    #   1942    bin
    #   2981    etc
    #   ...
    # listing the K bytes used under each file or directory.
    # It doesn't show other information, such as the
    # modification date or owner.
    # This program gets du's kbytes and filename, and merges
    # this info with other useful information for each file.
    #

    $files = join(' ',@ARGV);

    # The trailing pipe "|" directs command output
    # into our program:

    if (! open (DUPIPE,"du -sk $files | sort -nr |"))  {
        die "Can't run du! $!\n";
    }

    printf "%8s %-8s %-16s %8s %s\n",
        'K-bytes','Login','Name','Modified','File';
    while (<DUPIPE>) {
        # parse the du info:
        ($kbytes, $filename) = split;

        # Call system to look up file info like "ls" does:
        ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,
            $size,$atime,$mtime,$ctime)
            = stat($filename);

        # Call system to associate login & name with uid:
        if ($uid != $previous_uid) {
            ($login,$passwd,$uid,$gid,$quota,$comment,
                $realname,$dir,$shell) = getpwuid($uid);
            ($realname) = split(',',substr($realname,0,20));
            $previous_uid = $uid;
        }

        # Convert the modification-time to readable form:
        ($sec,$min,$hour,$mday,$mon,$myear) = localtime($mtime);
        $mmonth = $mon+1;

        printf "%8s %-8s %-16s %02s-%02d-%02d %s\n",
            $kbytes, $login, $realname,
            $myear, $mmonth, $mday, $filename;
    }

Demonstration Output

    K-bytes Login    Name             Modified File
      40788 c527100  Fred Flintstone  95-10-05 c527100
      32685 c565060  Peter Parker     95-10-05 c565060
      24932 c579818  Clark Kent       95-10-06 c579818
      15388 c576657  Lois Lane        95-10-06 c576657
       9462 c572038  Bruce Wayne      95-10-06 c572038
       8381 c517401  Eric McGregor    95-10-05 c517401
       7022 c594912  Asterisk de Gaul 95-10-05 c594912

12. Matching

The =~ operator performs pattern matching and substitution. For example, if:

    $s = 'One if by land and two if by sea';

    if ($s =~ /if by la/) {print "YES"}
    else {print "NO"}

    if ($s =~ /one/) {print "YES"}
    else {print "NO"}

    if ($s =~ /one/i) {print "YES"}
    else {print "NO"}

Patterns can contain a mind-boggling variety of special directions that facilitate very general matching. See Perl Reference Guide section 13, Regular Expressions. For example, a period matches any character (except the "newline" \n character).

    if ($x =~ /l.mp/) {print "YES"}

Parentheses () group pattern elements. An asterisk * means that the preceding character, element, or group of elements may occur zero times, one time, or many times. Similarly, a plus + means that the preceding element or group of elements must occur at least once. A question mark ? matches zero or one times. So:

    /fr.*nd/  matches "frnd", "friend", "front and back"
    /fr.+nd/  matches "frond", "friend", "front and back"
                but not "frnd".
    /10*1/    matches "11", "101", "1001", "100000001".
    /b(an)*a/ matches "ba", "bana", "banana", "banananana"
    /flo?at/  matches "flat" and "float"
                but not "flooat"

Square brackets [ ] match a class of single characters.

    [0123456789] matches any single digit
    [0-9]        matches any single digit
    [0-9]+       matches any sequence of one or more digits
    [a-z]+       matches any lowercase word
    [A-Z]+       matches any uppercase word
    [ab n]*      matches the null string "", "b",
                    any number of blanks, "nab a banana"

[^...] matches characters that are not "...":

    [^0-9]       matches any non-digit character.

Curly braces allow more precise specification of repeated fields. For example [0-9]{6} matches any sequence of 6 digits, and [0-9]{6,10} matches any sequence of 6 to 10 digits.

Patterns float, unless anchored. The caret ^ (outside [ ]) anchors a pattern to the beginning, and dollar-sign $ anchors a pattern at the end, so:

    /at/         matches "at", "attention", "flat", & "flatter"
    /^at/        matches "at" & "attention" but not "flat"
    /at$/        matches "at" & "flat", but not "attention"
    /^at$/       matches "at" and nothing else.
    /^at$/i      matches "at", "At", "aT", and "AT".
    /^[ \t]*$/   matches a "blank line", one that contains nothing
                          or any combination of blanks and tabs.

The Backslash. Other characters simply match themselves, but the characters +?.*^$()[]{}|\ and usually / must be escaped with a backslash \ to be taken literally. Thus:

    /10.2/       matches "10Q2", "1052", and "10.2"
    /10\.2/      matches "10.2" but not "10Q2" or "1052"
    /\*+/        matches one or more asterisks
    /A:\\DIR/    matches "A:\DIR"
    /\/usr\/bin/ matches "/usr/bin"

    /[-+]?\d*\.?\d*/      is the same as
    /[-+]?[0-9]*\.?\d*/

A simple \s specifies "white space", the same as the character class [ \t\n\r\f] (blank, tab, newline, carriage return,form-feed). A character may be specified in hexadecimal as a \x followed by two hexadecimal digits; \x1b is the ESC character.

A vertical bar | specifies "or".

    if ($answer =~ /^y|^yes|^yeah/i ) {
         print "Affirmative!";
    }

    #!/usr/local/bin/perl -w
    $s = 'There is 1 date 10/25/95 in here somewhere.';
    print "\$s=$s\n";
    $s =~ /(\d{1,2})\/(\d{1,2})\/(\d{2,4})/;
    print "Trick 1: \$1=$1, \$2=$2, \$3=$3,\n",
	  "         \$\`=",$`," \$\'=",$',"\n";
    
    ($mo, $day, $year) =
        ( $s =~ /(\d{1,2})\/(\d{1,2})\/(\d{2,4})/ );
    print "Trick 2: \$mo=$mo, \$day=$day, \$year=$year.\n";
    
    ($wholedate,$mo, $day, $year) =
        ( $s =~ /((\d{1,2})\/(\d{1,2})\/(\d{2,4}))/ );
    print "Trick 3: \$wholedate=$wholedate, \$mo=$mo, ",
        "\$day=$day, \$year=$year.\n";

Results of above:

    $s=There is 1 date 10/25/95 in here somewhere.
    Trick 1: $1=10, $2=25, $3=95,
             $`=There is 1 date  $'= in here somewhere.
    Trick 2: $mo=10, $day=25, $year=95.
    Trick 3: $wholedate=10/25/95, $mo=10, $day=25, $year=95.

Regular expressions are greedy. In the following example we try to match whatever is between "<" and ">" :

    #!/usr/local/bin/perl -w
    $s = 'Beware of <STRONG>greedy</strong> regular expressions.';
    print "\$s=$s\n";
    ($m) = ( $s =~ /<(.*)>/ );
    print "Try 1: \$m=$m\n";
    ($m) = ( $s =~ /<([^>]*)>/ );
    print "Try 2: \$m=$m\n";

    $s=Beware of <STRONG>greedy</strong> regular expressions.
    Try 1: $m=STRONG>greedy</strong
    Try 2: $m=STRONG

Homework: Parsing and Reporting

1. See preceding "Grade Book" example. Using the same "stufile" input, print a list of students ordered by family name, with any quoted nickname listed in place of the given name, and family name last. Produce output like this:

    Student-ID  Year  Name
    357913      JR    Thomas Jefferson
    246802      SO    Abe Lincoln
    212121      SO    Teddy Roosevelt
    123456      SR    George Washington

For an introduction to Common Gateway Interface, see http://hoohoo.ncsa.uiuc.edu/cgi/ and http://www.ncsa.uiuc.edu/SDG/Software/Mosaic/Docs/fill-out-forms/overview.html .

Example: Server Status Report

    c676828  q0  128.206.58.85       3  pico
    c676828  ftp UNKNOWN@128.20         -
    ccgreg   q1  monad.missouri         tcsh
    c552997  q5  mizzou-ts2.mis         telnet

Here's the program that produced it:

    #!/usr/local/bin/perl
    # Send error messages to the user, not system log
    open(STDERR,'<&STDOUT');  $| = 1;
  
    # Headers terminate by a null line:
    print "Content-type: text/html\n\n";
    $host = `hostname`;
    chop $host;
    $uptime = `uptime`;
    $w = `w -s -h`; 
    print <<BUNCHASTUFF;
    <HTML><HEAD>
    <TITLE>$host Status</TITLE>
    </HEAD><BODY>
    <H1>What's Happening at $host</H1>
    $uptime
    <PRE>$w</PRE>
    <HR>
    </BODY></HTML>
    BUNCHASTUFF
    exit;

Suppose user "bitman" wants to store this program on MU's SHOWME and SGI webservers, which use the "Apache" server software, configured for "CGI anywhere". If bitman doesn't already have a web directory, he should create one with these Unix commands:

    mkdir ~/www; chmod a+x ~ ~/www

    chmod a+rx ~/www/status.cgi

On the SHOWME server, the program would then be referenced as: http://www.missouri.edu/~bitman/status.cgi

Example: Web Form

Enter your ID Number:  
Enter your Name:          
Select favorite Color: 
redgreenblue

When something is entered and the submit button pressed, here is a resulting screen:

Perl Program to Generate and Process Form

    #!/usr/local/bin/perl
    # Send error messages to the user, not system log
    open(STDERR,'<&STDOUT');  $| = 1
    require "cgi-lib.pl";  # Get external subroutines
    print &PrintHeader;
    
    $script = $ENV{'SCRIPT_NAME'};
    $webserver = $ENV{'SERVER_NAME'};
    
    if (! &ReadParse(*input)) { &showform }
    else { &readentries }
    exit;

    sub showform {
    # If there is no input data, show the blank form

    print <<EOF;
    <HTML><HEAD>
    <TITLE>Form Example, Part 1</TITLE>
    </HEAD><BODY> 
    <H1>Web Form Example</H1>
    <P>(From http://$webserver$script)
    <FORM METHOD="POST" 
	ACTION=$script>
    <PRE>
    Enter your ID Number:  <INPUT NAME=idnum>
    Enter your Name:       <INPUT NAME=name>   
    Select favorite Color: <SELECT NAME=color>
    <OPTION>red<OPTION>green<OPTION>blue
    </SELECT>
    </PRE>
    To submit the query, press this button: 
    <INPUT TYPE=submit VALUE="Submit Request">
    </FORM>
    </BODY></HTML>
    EOF
    } # End of sub showform #
    
    sub readentries {
    # Input data was detected.  Echo back to form user.

    print <<EOF;
    <HTML><HEAD>
    <TITLE>Form Example, Part 2</TITLE>
    </HEAD><BODY>
    <H1>Results of Form</H1>
    <P>(From http://$webserver$script)
    <P>Your ID Number is $input{'idnum'}, your name is $input{'name'},
    and your favorite color is $input{'color'}.
    <HR>
    [<A HREF=$script>Try again</A>]
    EOF
    } # end of sub readentries #

Use the compiler -w switch to warn about identifiers that are referenced only once, uninitialized scalars, redefined subroutines, undefined file handles, probable confusion of "==" and "eq", and other things. This can be coded in the magic cookie first line:

    #!/usr/local/bin/perl -w

As you write your program, put in print statements to display variables as you proceed. Comment "#" them out when you feel you don't need to see their output.

CGI scripts require some special attention in testing.

MU's "showme" and "SGI" Web servers (www.missouri.edu and www.cclabs.missouri.edu) use the Apache "sucgi" facility. This causes CGI programs stored under your directory ~/www/ with file name ending ".cgi" to execute as your own ID. On some other Web server the script does not execute under your login ID! It executes under the ID of the Web server, typically as user "nobody".

Thus a script that works at the command line may fail under the Web server because:

    $ENV{'PATH'} .= ':~myid/bin:~myid/cgibin';
    push @INC,'~myid/lib/perl';

1. Testing "all-at-once" instead of incrementally, either bottom-up or top-down.
2. Optimistically skipping print scaffolding to dump values and show progress.
3. Not running with the perl -w switch to catch obvious typographical errors.
4. Leaving off $ or @ or % from the front of a variable, or omitting & when invoking a subroutine in Perl 4.
5. Forgetting the trailing semicolon.
6. Forgetting curly braces around a block.
7. Unbalanced (), {}, [], "", '', ``, and sometimes <>.
8. Mixing '' and "", or / and \.
9. Using == instead of eq, != instead of ne, = instead of ==, etc. ('White' == 'Black') and ($x = 5) evaluate as (0 == 0) and (5) and thus are true!
10. Using "else if" instead of "elsif".
11. Not chopping the output of backquotes `date` or not chopping input:

        print "Enter y to proceed: ";
        $ans = <STDIN>;
        chop $ans;
        if ($ans eq 'y') { print "You said y\n";}
        else { print "You did not say 'y'\n";}
    

12. Putting a comma after the file handle in a print statement.
13. Forgetting that Perl array subscripts and string indexes normally start at 0, not 1.
14. Using $_, $1, or other side-effect variables, then modifying the code in a way that unknowingly affects or is affected by these.
15. Forgetting that regular expressions are greedy, seeking the longest match not the shortest match.

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