G'day pldanutz,

I generally think it's better to pass around scalars than entire data structures, so returning a reference to that hash would be my preference.

If you wrapped $h{$k} in @{...}, you could dispense with if (! $h {$k}) { $h {$k} = [] } entirely.

Even if you're using 5.012 for other reasons elsewhere in your code, I see little, if any, value in using each here. It's easy to write it for any Perl5.

Putting all that together, you could code occurrences() like this instead:

sub occurences {                                                    
    my %h;          
    my $i = 0;
    push @{$h{$_}}, $i++ for @_;
    return \%h;
}
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If you were looking for a golf answer, you could start with the following and then proceed to remove whitespace until illegibility was achieved. :-)

sub occurences { my ($i, %h) = 0; push @{$h{$_}}, $i++ for @_; \%h }
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Here's my tests: