japhy,
When I am doing this without using an existing wheel, it usually looks something like:

sub thing {
    my $self = shift @_;
    croak "Incorrect number of arguments" if @_ % 2;
    my %arg = @_;
    for (@known_args) {
        # handle $arg{$_};
        delete $arg{$_};
    }
    if (keys %arg) {
        my $bad = join ' ', keys %arg;
        croak "The following args are invalid: $bad";
    }
    # ... remainder of thing
}
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