sub mysub {
    %params = %{ +shift };
    my %known_options = ( 'this' => 1, 'that' => 2 );
    PARSE: foreach my $param ( keys %params ) {
        exists $known_options{$param} && 
            do {    #something; 
                    next PARSE; };
        die "Usage: I only know how to 'this' and 'that'\n";
    }
    # Rest of sub here...
}