Looks like all of your attempts are broken. I'd expect the resulting string to be aA,cd, and not to lose all its commas.

I'd go for a variant of attempt 3:

    my $repl = '$1';
    my $str = "a,A,c,d";
    s/([a-z]),/$repl/eeg;  # Note the double 'e' modifier
    say $str;
    __END__
    aA,cd
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No need to turn off strictness.

Now, if you don't want string evals in the loop, I presume you want that because of performance. But you replace that with a subroutine call - I'd have to see a benchmark to be certain that is faster.

But what you could do is (string) eval your entire loop:

    my $repl = '$1';
    eval <<"EOT";
        while (1) {
            \$str =~ s/([a-z]),/$repl/g;
        }
    EOT
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