$ perl -Mstrict -Mwarnings -e '
    use Data::Dumper;

    sub occurences {                                                    
        my %h;          
        my $i = 0;
        push @{$h{$_}}, $i++ for @_;
        return \%h;
    }
                                               
    my $h = occurences(2, 1, 3, 4, 3, 4, 4, 5);
    print Dumper $h;
'
$VAR1 = {
          '1' => [
                   1
                 ],
          '3' => [
                   2,
                   4
                 ],
          '2' => [
                   0
                 ],
          '4' => [
                   3,
                   5,
                   6
                 ],
          '5' => [
                   7
                 ]
        };