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

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