PLPerl Trigger to update text search

I am trying to update a tsvector field through a plperl trigger.

$_TD->{new}{text_search} = to_tsvector('pg_catalog.english', 'text1 text2');

but plperl does not seem to like that... 
ERROR:  Undefined subroutine &main::to_tsvector called 

anyone done that and could help me out if that is actually possible?

Thanks in advance

Alex