#!/usr/bin/env perl
use strict;
use warnings;
use 5.016;

for (<DATA>) {
    chomp;
    print "Match: $_\n" if /(?=name=bob)(?!flags=.*?cat.*?;)/;
}

__DATA__
name=bob;flags=human;age=10
name=tiddles;flags=cat,black;age=3
name=bob;flags=cat,white;age=6