use Data::Validate::IP qw(is_ipv4);

my $ip_w_mask = '10.3.2.0/28';

my ($ip, $mask) = m{^(.*)/(\d+)$}
   or die("Incorrect format: Mask not found\n");

is_ipv4($ip)
   or die("Incorrect format: Malformed IP address\n");