For some reason i've always wanted to write a polymorphic perl script. Of course an example was quickly found on perlmonks (Self-Modifying-Perl-Script) and chromatic's code worked perfectly from the command line, but i couldn't make it work via CGI so set out to learn more perl and write my own.

I ended up having to spawn a child script that modifies it's parent. Oddly, i don't really understand a key area of this script i wrote (and got working by fiddling around with what worked). Here's what i mean (full source at eof).

After a child is opened, written to and closed:

open(X,">$time.pl") or die "$!";
print X $child; 
close(X);
[download]

The child is executed by the parent,

system "perl $time.pl";
[download]

The last thing the child does is (successfully) call the parent via CGI with a parameter like this:

print "Location: $parent_url?d=1\n\n";
[download]

but the next line in the parent successfully deletes the child!

unlink "$time.pl";
[download]

How does the child get deleted by the parent after modifying it and printing it's location? Btw this was developed and tested on Win98 with Activestate5.

#!perl -w

use strict;
use CGI qw(:standard);

my$url = url(-relative=>1);
my$time=time();
my$d = param('d');
if($d eq '1'){&show;exit}

eval("seek DATA,0,0;"); undef$/; $_=<DATA>; # read self into $_
$_ =~ m/__END__\n(\d.*)/; my$num=$1; $num++; # find target in $_ and c
+hange

# build child script with $url and $num (to open, change, save, and ru
+n parent) 
my$child = "#!perl -w\n\nuse CGI qw(:standard);\nopen(Z,\"+>>$url\") o
+r die \"\$!\";\nundef\$/;\n\$_=<Z>;\n\$_=~s|__END__\\n(\\d.*)|__END__
+\\n$num|o;\nseek Z, 0, 0;\ntruncate Z, 0;\nprint Z \$_;\nclose(Z);\np
+rint \"Location: $url?d=1\\n\\n\";"; 

open(X,">$time.pl") or die "$!"; # create and 
print X $child; close(X); # print and close
system "perl $time.pl"; # and run and
unlink "$time.pl"; # delete child

sub show{
print header,start_html(-title=>'polymorphic perl',-bgcolor=>'#000000'
+);
while(<DATA>){ print qq~<a href="$url"><font size="+5" color="#FFFFFF"
+>$_</font></a>~}}

exit;
__END__
0
[download]

And here is a child (pp.pl is whatever url() in the parent returned, and the 8 in the regex is the value of $num):

#!perl -w

use CGI qw(:standard);
open(Z,"+>>pp.pl") or die "$!";
undef$/;
$_=<Z>;
$_=~s|__END__\n(\d.*)|__END__\n8|o;
seek Z, 0, 0;
truncate Z, 0;
print Z $_;
close(Z);
print "Location: pp.pl?d=1\n\n";
[download]

thanks - epoptai

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