#!/usr/bin/perl
use strict;

my $output = "";

open(FILE, "file.txt") || die "$!";
while(<FILE>)
{
  $output .= $_;
}
close FILE;

$output =~ s#(http://[^\!\"\£\$\^\*\(\)\{\}\[\]\;\:\'\@\,\<\> ]+)#\<a href="$1" target="_blank"\>$1\</a\>#gis;

print $output;

##</code><code>##

The quick brown fox at http://fish.org would like to go home and http://see
through.com/pants/foo/bar what there is to eat

##</code><code>##

The quick brown fox at <a href="http://fish.org" target="_blank">http://fish.org</a> would like to go home and <a href="http://see
through.com" target="_blank">http://see
through.com</a> what there is to eat