You can do your substitusion this way with the following code.

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

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

I know this is a bit brute force but it does work. Hope this helps. Edit:Hey I went back and changed the $a $b to other varables and the regular way works. ex.

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