for ("I am a fish","I am a nonfish","I am a nonfish","fish I am a") {
    print "Start> $_\n";
    if (/\GI\s*/g) {
        print "I\n";
        if (/\Gam a\s*/g) {
            print "am a\n";
            if (/\Gfish/g) {
                print "Yah! I am a fish\n";
            }
        }
    }
}

##</code><code>##

$_="aiutino aiutino aiutino fnorbley";
/ \b aiutino \b \s* (?{ warn "Argh!" }) fnorbletrx /gix;