sub occurs{ 
    my %h; 
    push @{ $h{ $_[$_] } }, $_ for 0 .. $#_; 
    return \%h 
};;

pp occurs( 2, 1, 3, 4, 3, 4, 4, 5 );;
{ 1 => [1], 2 => [0], 3 => [2, 4], 4 => [3, 5, 6], 5 => [7] }