my $repl = '$1';
    my $str = "a,A,c,d";
    s/([a-z]),/$repl/eeg;  # Note the double 'e' modifier
    say $str;
    __END__
    aA,cd

##</code><code>##

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