#!/usr/bin/perl
use CGI;
use strict;

my $q = CGI->new;

my $string = $q->param('content');  # the posted string.
$string =~ s/\r?\n$//s; # remove trailing CRLF

if ($string =~ /foo/) { # decide what to do depending on input
    print $q->redirect('http://host.tdl/some/other/uri');
}
else {
    print $q->header,$q->start_html;

    if($string) {
        # print "<p>You posted: $string\n";   # <-- bad
        # what oxone said below - avoid XSS attacs
        print "<p>You posted: ",$q->escapeHTML($string),"\n"; # <-- be
+tter
    } else {
    print <<EOH;
<form action="/cgi-bin/posttest.cgi" method="POST">
<textarea name="content" cols=40 rows=4>Enter data here.
</textarea>
<input type="submit" value="Process">
</form>
EOH
    }
    print $q->end_html;
}
[download]

update: fixed XSS vulnerability. Thanks, oxone.

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}