$a = 'aa';
$a =~ s/a/b/;
print $a

##</code><code>##

$a = 'aa';
$b = 's/a/b/';#or $b = qr(s/a/b/)
$a =~ $b;
print $a

##</code><code>##

($a) = map {eval $b} $a;