Please provide an example of how you would handle this contrived and simple case, as a teaser presented in Perl 6:

Most of the time I would do something like:

sub convert {
    my ( $from, $to, $thing ) = @{ {@_} }{ qw( from to thing ) };
    ...
}
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With in-house code I find that my test suite reveals where I accidentally miss required parameters, so I only add explicit code inside the subroutine to check if it's an API I'm presenting to others.