Perhaps I have misread the replies, but I didn't see anyone suggest returning a hashref from the sub:

# Two hashes:
%myhash;
%b;
# Merge them
%myhash = %{mergetwohashes(\%myhash, \%b)};

sub mergetwohashes {
    return { %{$_[0]},  %{$_[1]} };
}
[download]

Though this isn't as efficient as the other solutions, it may be simple enough for what you are after.

-QM
--
Quantum Mechanics: The dreams stuff is made of