#!/usr/bin/perl -w

use strict;

chomp(my $num = <DATA>);

print "My counter is set to $num.\n";

$num++;

{
    local *FILE;
    local $/ = "\n__END__";
    open (FILE, "counter.pl") || die "Can't open my file for reading: 
+$!\n";
    chomp(my $file = <FILE>);
    close FILE;
    open (FILE, ">counter.pl") || die "Can't open file for writing: $!
+";
    print FILE $file, "\n__END__\n$num";
    close FILE;
}
__END__
0
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Here's something I whipped up for fun:

#!/usr/bin/env perl -pi.orig
use warnings;
use strict;
BEGIN {
    @ARGV = $0;
    while (<>) {
        do { print; last; } if /^__DATA__$/;
    } continue {
        print;
    }
}
tr/A-Z/N-ZA-M/;
tr/a-z/n-za-m/;
__DATA__
Guvf vf n cebtenz gung zbqvsvrf vgfrys va cynpr.
Vg ebg13 rapbqrf vgf bja __QNGN__ frpgvba jura lbh eha vg.
Nalguvat orsber gur __QNGN__ frpgvba vf yrsg hazbqvsvrq.
Lbh jvyy frr gur sbyybjvat qvntabfgvp reebe, juvpu lbh pna vtaber:

    -v hfrq jvgu ab svyranzrf ba gur pbzznaq yvar, ernqvat sebz FGQVA.
[download]

$end = 0;
$var = 1;
$code='print "test $code";
$end = 1 if ($var >= 2);
$var++;
eval ($code) if ($end == 0);';
eval($code);
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for what it's worth, i can not conceive, other than as a test scenario, for a reason to make a script self modifying in such a manner. Look at the program's design from a logical standpoint. If it makes sense for an external file to contain your configuration information, or your test data, or something NOT internal to the logic of your code, then put it an external file! Remeber, we want to make our code as readable and accessible to people other than ourselves. Anyone who ever has to look at your code somday down the road will thank you for it. Let's make sure that this isn't a case of us being too clever for our own good!

#!/usr/bin/perl

# lives tracks the number of times the program has run.
# the line number is stored for later use in an array
# called @program, so if consensual hackers decide to 
# rearrange, the program can still find the variable.

$lives = 0; $line{lives} = (__LINE__ - 1);

# The source code of the file is read into @program
open( THIS, "$0" );
 @program = <THIS>;
close( THIS );

# The line number from earlier points at the line of code
#  storing the variable '$lives'.
# Render the line and store it back into @program over the 
#  old line of code. 
$program[ $line{lives} ] =
 '$lives = ' . ($lives+1) . 
  '; $line{lives} = (__LINE__ - 1);' . "\n";

# Output the program's slightly modified source 
#  code over the old file.
open( THIS, ">$0" );
 print THIS @program;
close( THIS );

# Clear the screen buffer and 'exec' the modified source.
system( 'clear' );
exec( $0 ); # exec() does not return to this program.

print "This does not get displayed.";
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