my $new = 'blah';
my $string = 'asdfjkjl';
my $find = '(asd)';
my $replace = '$1' . $new;

replace ($string, $find, $replace);

sub replace {
  my $string = shift;
  my $left = shift;
  my $replace = shift;
  $string =~ s/$left/$replace/e;
  print $string;
}