$a = 2;
$b = "-";
$base = "ABCABC";

$stemp = "\/" . "ABC" x $a . "\/";
$regxp = eval 'qr' . $stemp;
$base =~ s/$regxp/$b/g;

##</code><code>##

$base =~ s/[ABC]{$multi}/$replace/g;