Executing a .cgi from the command-line (as the user who has access to the db), I get
install_driver(Pg) failed: Can't locate DBD/Pg.pm in @INC (@INC contains: /Library/Perl/5.12/darwin-thread-multi-2level /Library/Perl/5.12 /Network/Library/Perl/5.12/darwin-thread-multi-2level /Network/Library/Perl/5.12 /Library/Perl/Updates/5.12.4 /System/Library/Perl/5.12/darwin-thread-multi-2level /System/Library/Perl/5.12 /System/Library/Perl/Extras/5.12/darwin-thread-multi-2level /System/Library/Perl/Extras/5.12 .) at (eval 4) line 3. Perhaps the DBD::Pg perl module hasn't been fully installed, or perhaps the capitalisation of 'Pg' isn't right.
cpan -D DBD::Pg shows
DBD::Pg
Postgres Driver for DBI T/TU/TURNSTEP/DBD-Pg-2.19.2.tar.gz /opt/local/lib/perl5/site_perl/5.12.4/darwin-thread-multi-2level/DBD/Pg.pm Installed: 2.19.2 CPAN: 2.19.2 up to date DBD::Pg Developers (DBDPG) dbd-pg@perl.org
Trying to export above path by PERL5LIB or PERLLIB has no different outcome.
The .cgi references /usr/bin/perl, which tells
This is perl 5, version 12, subversion 4 (v5.12.4) built for darwin-thread-multi-2level (with 2 registered patches, see perl -V for more detail)
I like to permanently set the module path to the one where DBD::Pg is installed (/opt/...). I wonder why that is not mentioned by the error outputs directory listing.
I installed DBD::Pg via cpanm.
I use Mac OS X Mountain Lion.
You appear to have two different builds of Perl installed. The one with which you are running your script is different than the one used to run cpan. To install DBD::Pg with the correct Perl, use
perl -MCPAN -e'install DBD::Pg'
Replace perl with the path to the Perl used to run your script, if necessary.