open (FH, "< $parent_index") or die "could not open file $parent_index for reading:$!";

##</code><code>##

 open (FH, '<', $parent_index) or die "could not open file $parent_index for reading:$!";

##</code><code>##

 open (my $fh, '<', $parent_index) or die "could not open file $parent_index for reading:$!";