Undefined subroutine &main::spi_prepare

I have a PL/Perl stored procedure that's worked fine on my development
boxes but is suddenly failing when installed on a test machine. Both my
test machines and the new machine are RHEL AS5. The differences are the
test machine is 64-bit, and running Postgres 8.1.18 instead of my
development machine which is 32-bit and running Postgres 8.2.7.

The error in the postgresql log is:

ERROR: error from Perl function: Undefined subroutine
&main::spi_prepare called at line 2.
LOG: unexpected EOF on client connection

My first thought was that the postgresql-pl package wasn't installed,
but it appears to be there. Does anyone else know what sort of
installation/configuration error could cause a failure like this?

Thanks.
-Tony