The attempt number three is still invalid. Try it with my $repl = '$1-$1-'; The attempt four fails if the replacement contains a doublequote.

#my $repl = '$1-$1-';
use strict;
use warnings;


#my $repl = '$1';
my $repl = '$1"{$1-';
{
    print "attempt 1 ";
    my $str = "a,A,c,d";
    $str =~ s/([a-z]),/$repl/ig;
    print "yields: $str\n";
}

{
    no strict 'refs';
    print "attempt 2 ";
    my $str = "a,A,c,d";
    $str =~ s/([a-z]),/$$repl/ig;
    print "yields: $str\n";
}

{
    print "attempt 3 ";
    my $str = "a,A,c,d";
    $str =~ s/([a-z]),/eval "$repl"/ieg;
    print "yields: $str\n";
}


my $code = 'sub {$_[0] =~ s/([a-z]),/'. $repl . '/ig}';
my $re = eval ($code);
if ($@) { print "attempt 4 failed: $@" }
else {
    print "attempt 4 ";
    my $str = "a,A,c,d";
    $re->($str);
    print "yields: $str\n";
}

{
    print "attempt 5 ";
    (my $repl = $repl) =~ s/"/\\"/g;
    my $str = "a,A,c,d";
    $str =~ s/([a-z]),/qq{"$repl"}/igee;
    print "yields: $str\n";
}
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