Its giving error "Use of uninitialized value in numeric comparison (<=>)"

That's because this is almost certainly not doing whatever you think it's doing:

$ab = "\$a";
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That's taking the value of $a and appending it to a backslash and making it the value of $ab, so "11" becomes "\11". I'm guessing that you're trying to make $ab a reference to $a, but to do that you'd need to leave out the quotes, and that would also change the later code.

Personally, if I wanted to have a toggle between two different ways to sort, I'd do it like this (unless the sort comparison is very complex, in which case it should be in a separate subroutine anyway):

sub sort_array_by_column_asc_or_desc {
  my( $array, $column, $order ) = @_;
  if( $order eq 'desc' ){
    return sort { put_descending_sort_comparison_here } @$array;
  } else {    # default to ascending sort
    return sort { put_ascending_sort_comparison_here } @$array;
  }
}
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Note: the else there isn't necessary, but I like it because it makes the choice obvious.