use Params::Validate qw(validate);

sub convert {
    my %args = validate(@_, { from  => 1,
                              to    => 1,
                              thing => 1});
    # ...
}
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Constructing a more interesting example would require knowing what kind of data should be in the parameters. Params::Validate has some decent built-in type-checks and makes rolling your own easy.

-sam