I believe the error in your ways is not checking to see if the match succeeded before using the match capture variable values. Instead of this:

$dorient =~ /^([^_]+)_/;
$rs[2] = $1;
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You should be checking to ensure that the match succeeded:

if( $dorient =~ /^([^_]+)_/ ) {
  $rs[2] = $1;
}
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This should be true of almost any time you rely on $1; you should first check to see if $1 is meaningful by verifying the success of its corresponding regex match.

The behavior is described in perlop wherein it states: "NOTE: failed matches in Perl do not reset the match variables...."