$sub_string =~ s/$sub_in/$sub_out/eeg;

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";

  my @info = $sub_string =~ /$sub_in/;
  $sub_string =~ s/$sub_in/$info[$sub_out]/g;
  print "erg: $sub_string \n";
}