sub on (&$$) {
        my($f1,$f2,$l)=@_;
        
        local $_=$$l;
        if (&$f1) {
                local *_=$l;
                &$f2($l);
        }
}

sub action (&) { shift }

$a="abba is foo";
on { /foo/ } action { tr/ab/ba/; },\$a;
print "$a\n";