Your question isn't that clear, but if you want column_exists to return a particular element, or the zeroth element if that element is undef, then you could write the following:

sub column_exists
{
  my ( $ArrayRef, $Column ) = @_;
  return ( $ArrayRef->[$Column] || $ArrayRef->[0] );
}
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If you want the calling code to die if the value is -1, the following will probably work:

  my $label_colnum = &column_exists( \@data, 1 )
  if ( $label_colnum == -1 ) { die "Something bad!"; }
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--t. alex

"Here's the chocolates, and here's the flowers. Now how 'bout it, widder hen, will ya marry me?" --Foghorn Leghorn