unless
(
    $ip =~ m/^(?:[1]?\d\d?|2[0-4]\d|25[0-5])\.
              (?:[1]?\d\d?|2[0-4]\d|25[0-5])\.
              (?:[1]?\d\d?|2[0-4]\d|25[0-5])\.
              (?:[1]?\d\d?|2[0-4]\d|25[0-5])$
            /x
)
{
    # do something
}

##</code><code>##

if ( $ip =~ /^0.0.0.0$/ )
{
    # do something
}