Since it hasn't been mentioned:

There's a Net::IPv4Addr module available on CPAN that parses IP addresses and checks legality. It'll also handle addr+netmask pairs like "11.22.33.44/24" or "11.22.33.44/255.255.255.0". The relevant snippet to extract a legal address would be:

use Net::IPv4Addr qw( :all );
if ($ip = ipv4_checkip($str)) {
      # do something
}
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Unfortunately, most of the other useful-looking methods croak instead of returning undef on failure, so the documentation says you have eval 'em to handle strings with malformed addresses...