Historical version. PostgreSQL 7.3 has reached end of support. See the current manual.
21.5. Missing Features
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). There's presently no way for them to share global variables, either.
PL/Perl cannot be used to write trigger functions.
DBD::PgSPI or similar capability should be integrated into the standard PostgreSQL distribution.
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Read the upstream documentation. For corrections, first check the current manual.