Hi,
your regex was far too sophisticated IMHO.

#!/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=dsc0
+3598vt7.jpg]'
                 . '[IMG]http://img207.imageshack.us/img207/2964/dsc03
+598vt7.jpg[/IMG][/URL]';


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

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