Depends on your amitiousness.

$regexp = qr/a(b+)c/;
$subst  = 'c$1a';

my @a = $string =~ $regexp;
$subst =~ s/\$(\d)/$a[$1-1]/ge;
$string =~ s/$regexp/$subst/;
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Warning - untested. I know I've used code similar to this in the past with a limited amount of success. (i.e., it worked for me for what I needed it to work for, and this is not a cut&paste job of that code.)