my $Y = [ qw( a b a) ];
  
  my $YPos;
  my $index;
  for ( $index = 0 ; $index <= $#$Y ; $index++ ) {
    push ( @{ $YPos->{$Y->[$index]} }, $index );
  }

  # now $YPos should contain
  my $result = {
    a => [ 0, 2 ],
    b => [ 1 ],
  };