You can do the second part without an eval but with an ee modifier, like this

$ perl -le '
> $str = q{aaabbbaaa};
> $repl = q{$1};
> $str =~ s{a+(b+)a+}{$repl}ee;
> print $str;'
bbb
$
[download]

The first e executes the $repl resulting in $1, the second e executes the $1 resulting in whatever was captured, "bbb" here.

Cheers,

JohnGG