sub Whatever
{
    # Optional test of the arguments. Good for catching programmer errors. 
    # Not that anyone here ever makes any. 8-)
    die 'Takes a hash ref' if @_ and grep { "HASH" ne ref } @_;

    # Set up the defaults here:
    my %args = ( KEY1 => 'default value', KEY2 => 'etc...' );

    # And process the args.
    my $index = @_; # Intentionally reverse the order.
                    # The first hash thus has precedence.
    while( --$index >= $[ )
    {
        $args{$_} = $_[$index]->{$_} for keys %{$_[$index]}
    }

    # Do stuff with %args.
}