I've used Net::Netmask a few times. The new2 method looks ok. Looks like your trying to setup a Net::Netmask object using wild card bits instead of a subnet mask. I haven't done that before with Net::Netmask, but it looks like things went ok until line 17 of the input file. You also are using new2 which should return undef if fails. You could try adding a error check in there. Maybe the input isn't what you expected. my $acl_block = new2 Net::Netmask( $b ) or die $Net::Netmask::error to see if maybe the object isn't being created correctly and possibly why.