use strict;

my $string = "testx_testy";
my $in = "(testx_)(testy)";

my $out = '$1';

test("$string","$in","$out");

sub test
{
my $sub_string = shift;
my $sub_in = shift;
my $sub_out = shift;

print "$sub_string <=> $sub_in <=> $sub_out \n";

$sub_string =~ s/$sub_in/$sub_out/g;
print "erg: $sub_string \n";
}