As davido says, checking that the match succeeded is almost always the right solution. If for some reason you don't want to do that, easiest way is to throw a cheap guaranteed match in before the real match:

  "" =~ /(?=)/;  # reset match vars
  $dorient =~ /^([^_]+)_/;
  $rs[2] = $1;   # or undef if no match
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Hugo