use strict;
use warnings;


my $repl = '$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 "eval failed: $@" }
{ 
	print "attempt 4 ";
	my $str = "a,A,c,d";
	$re->($str);
	print "yields: $str\n";
}

__END__

attempt 1 yields: $1$1$1d
Use of uninitialized value in substitution (s///) at zwomp.pl line 17.
attempt 2 yields: d
attempt 3 yields: aAcd
attempt 4 yields: aAcd

##</code><code>##

URL: 
	domain: 
		match: http://(\w+)\.(\w+)\.
		replace: $2

##</code><code>##

apply s/http://(\w+)\.(\w+)\./$2/ to 'URL' to produce 'domain'