my $post_message = '[URL=http://img207.imageshack.us/my.php?image=dsc0
+3598vt7.jpg][IMG]http://img207.imageshack.us/img207/2964/dsc03598vt7.
+jpg[/IMG][/URL]';

print "before: ",$post_message,"\n";
$post_message =~ s#\Q[URL=\E([^]]+)\Q][img]\E([^]]+)\Q[/img][/URL]\E#$
+2#gix;
print "after: ",$post_message,"\n";
[download]

prints

before: [URL=http://img207.imageshack.us/my.php?image=dsc03598vt7.jpg]
+[IMG]http://img207.imageshack.us/img207/2964/dsc03598vt7.jpg[/IMG][/U
+RL]
after: http://img207.imageshack.us/img207/2964/dsc03598vt7.jpg
[download]

Regards,
svenXY