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

# The DATA filehandle is just a an open handle on the current 
# file. The current file is just the $0 variable (so long as 
# you haven't changed it)

# main part of script ...

my $count;
while(<DATA>){ $count = $_ ; print "$count\n";}
$count++;

# then redo the count update:
rewrite_count($count);

sub rewrite_count {
   my $count = shift;
   open(SELF,"+<$0")||die $!;
   while(<SELF>){last if /^__END__/}
   truncate(SELF,tell SELF);
   print SELF $count;
   close SELF;
}

__END__
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