The following features are currently missing from PL/Perl, but they would make welcome contributions.
PL/Perl functions cannot call each other directly (because they are anonymous subroutines inside Perl).
SPI is not yet fully implemented.
If you are fetching very large data sets using spi_exec_query, you should be aware that these will all go into memory. You can avoid this by using spi_query/spi_fetchrow as illustrated earlier.
A similar problem occurs if a set-returning function passes a large set of rows back to PostgreSQL via return. You can avoid this problem too by instead using return_next for each row returned, as shown previously.