=head2 perl2yaml($perlvar, $optional_paramshashref)

Given an input C<$perlvar> (which can be a simple scalar or
a nested data structure, but not an object), it will return
the equivalent YAML string. In C<$optional_paramshashref>
one can specify whether to escape unicode with
C<'escape-unicode'=E<gt>1>. Prettify is not supported yet.
The output can fed to L<Data::Roundtrip::yaml2perl>
for getting the Perl variable back.