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
conversionfunction 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'twork, 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"
atline 2. 

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

Thanks,

David