Something like this:

my $c = 'aaabbbaaa';
my $d = 's/b/c/g';
eval "\$c =~ $d";
die "Failure: $@\n" if $@;
print $c, "\n";
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Depending on where your $d is coming from, this could expose you to code injection. Be cautious.

Also notice I refrained from using $a and $b. This is usually a good idea, as the use of $a and $b can get confusing since sort uses special global versions of $a and $b also.