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

use re "debug"; # will help you understand your regexes

my $post_message = '[URL=http://img207.imageshack.us/my.php?image=dsc03598vt7.jpg]'
                 . '[IMG]http://img207.imageshack.us/img207/2964/dsc03598vt7.jpg[/IMG][/URL]';


my @fields = $post_message =~ m#\Q[URL=\E([^]]+)\Q][img]\E([^]]+)\Q[/img][/URL]\E#gix;
##                                         ^^^ one or more of not ']'
##                                         should be enough here.

print join("\n", @fields, "\n");