In Perl 6, you will be able to use assuming:

sub print_me ($arg = "Hello World!\n") {
    print $arg;
}

%hash = (
    no_arg   => &print_me,
    with_arg => &print_me.assuming(arg => "Hello World, again!\n")
);

%hash{no_arg}();
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Juerd # { site => 'juerd.nl', plp_site => 'plp.juerd.nl', do_not_use => 'spamtrap' }