I like to use references to the hash, but you can always just pass it to and from straight.

Perl will flatten your hash into a normal list. Luckily, if you're expecting this, you can capture it in a normal hash again.

my %values = (
    name => 'chromatic',
    rank => 'saint',
    style => 'obnoxious',
);

my %rev_values = do_something(%values);

foreach my $key (keys %rev_values) {
    print "$key => $rev_values{$key}\n";
}

sub do_something {
    my %values = @_;
    foreach my $key (keys %values) {
        print "$key => $values{$key}\n";
        $values{$key} = reverse $values{$key};
    }
    return %values;
}
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A reference is conceptually more complicated, but it solves issues like parameter positioning and the trickiness involved in converting and copying hashes:

my %values = (
    foo => 1,
    bar => 2,
    baz => 3,
);

my %rev_values = do_something(\%values);

sub do_something {
    my $h_ref = shift;
    my %backwards;
    foreach my $key (keys %$h_ref) {
        print "$key => ", $h_ref->{$key}, "\n";
        $backwards{$h_ref->{$key}} = $key;
    }
    return \%backwards;
}
[download]