You can either return a reference to a hash, which probably you don't since you assign to %acls which happens to be a hash, not a hashref, or return a flattened list, then assign it to a hash. But it seems that it is not what you want. Just remember that just like an array is not a list, and the difference is (in a technically imprecise terminology) fundamentally that "the former has a name", the same happens with hashes: a hash in list context is flattened. OTOH each and keys expect a hash, so you must give them a hash, full of its name. But if it is anonymous, then call it by its ehm anonymous name, by dereferencing:

#!/usr/bin/perl -l

use strict;
use warnings;

package Module;

sub new {
    bless { ACLS => {foo => 1, bar => 2} }, shift;
}

sub acls {
    my $self=shift;
    $self->{ACLS}
}


package main;

my $mod=Module->new;
while (my @stuff=each %{ $mod->acls } ) {
    print "@stuff";
}

__END__
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