Quick and dirty one line example. you need to escape the [] in the substitution.

 
perl -e '$post_message="[URL=http://img207.imageshack.us/my.php?image=
+dsc03598vt7.jpg][IMG]http://img207.imageshack.us/img207/2964/dsc03598
+vt7.jpg[/IMG][/URL]"; $post_message =~ s|^.*?\Q[img]\E([\?\%\:\/a-zA-
+Z0-9_\-\.]+)\Q[/img]\E.*$|\[img\]$1\[/img\]|sig;print "$post_message\
+n";'
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