Use UNKNOWN with PL/Perl spi_prepare()?

Hackers,

I tried this:

try=# create or replace function try() returns void language plperl as $$
spi_prepare('select length($1)', 'unknown');
$$;
CREATE FUNCTION
try=# select try();
ERROR: error from Perl function "try": failed to find conversion function from unknown to text at line 2.

I could figure out no way to specify an that a data type is unknown (as opposed to known to be "unknown"). Specifying 0 doesn't work, either:

try=# create or replace function try() returns void language plperl as $$
spi_prepare('select length($1)', 0);
$$;
CREATE FUNCTION
try=# select try();
ERROR: error from Perl function "try": syntax error at or near "0" at line 2.

Is there a way to do this? If not, should there be?

Thanks,

David