Trapping PL/Perl spi_query_exec errors

This is probably a really basic question, but how do I trap when
spi_query_exec returns an error in PL/Perl?  I would expect to be able
to use the return value to determine an error condition, however I get
the message:

ERROR:  error from Perl function: relation "foo" does not exist at line 7.

where line 7 looks like:

my $rv = spi_query_exec('select count(*) from foo');

Alternatively, I would expect to be able to trap it with eval, however
the trusted PL/Perl does not support eval.

Thanks!