I use a slightly different, and probably slower, strategy for default arguments. I like it though, because it allows me to pass in multiple hashrefs. So it goes something like:

sub Whatever
{
    # Optional test of the arguments. Good for catching programmer err
+ors. 
    # 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.
}
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Also, instead of saying keys, you could use an array of legal key values (using uc or lc to handle case issues) and then overwrite $arg{$legalkey} if it exists in the passed in hash.

Note that I keep saying "passed in hash", what I should say is "hash that the passed in hashref references", but I'm lazy. Then again I added this comment... sigh