When I used the $_ convention I came up with this:

/(21/4/2001 - 21:00:16) --- Intruder Detection Event - [Remote
=~ m~((\d{1,3}\.){3}\d{1,3})~)/: unmatched [] in regexp at C:\hack\hac
+k.pl line
12, <HACK> line 79.
[download]

Thanks for your help kschwab.