sub printh {
    tie my %x, 'Tie::IxHash', @_;  # Recreate the hash. Expensive
    warn $_ for keys %x;
}

printh(%h);  # Flatten the hash

##</code><code>##

sub printh {
    my ($x) = @_;
    warn $_ for keys %$x;
}

printh(\%h);  # Pass a reference