DB<112> @Y=qw(a b a)
 => ("a", "b", "a")

  DB<113> %Ypos=(); $i=0; 
 => 0

  DB<114> push @{ $Ypos{$_} },$i++ for @Y 
 => ""

  DB<115> \%Ypos
 => { a => [0, 2], b => [1] }