This may be something like what you want. It's usually best for maintainability and comprehensibility to build up complex regexes from simpler components. And then there's also Regexp::Common (see esp. Regexp::Common::net).

c:\@Work\Perl\monks>perl -wMstrict -le
"my $octet = qr{ 25 [0-5] | 2 [0-4] \d | [01]? \d [0-9]?   }xms;
 my $ip    = qr{ (?<! \d) $octet (?: \. $octet){3} (?! \d) }xms;
 my $addr  = qr{ $ip / \d{1,2} (?! \d) }xms;
 ;;
 my $s = 'x 0.0.0.0/0 y 1.23.234.1/1 z 255.25.2.2/99 '
       . '9999.9.9.999/1 z 9.9.9.9/999'
       ;
 ;;
 my @addrs = $s =~ m{ $addr }xmsg;
 printf qq{'$_' } for @addrs;
 print '';
 ;;
 ;;
 use Regexp::Common qw(net);
 ;;
 my @common = $s =~ m{ (?<! \d) $RE{net}{IPv4} / \d{1,2} (?! \d) }xmsg
+;
 printf qq{'$_' } for @common;
"
'0.0.0.0/0' '1.23.234.1/1' '255.25.2.2/99'
'0.0.0.0/0' '1.23.234.1/1' '255.25.2.2/99'
[download]

Update: Added Regexp::Common::net example.