My guess would be that the user the webserver runs under (often www-data) has no permissions to access /dev/ttyS0  (so the call to Device::SerialPort->new(...) fails and $pbx is undefined).

For example, on my system, /dev/ttyS0  is

$ ls -l /dev/ttyS0
crw-rw---- 1 root dialout 4, 64 2009-09-06 04:09 /dev/ttyS0
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and Apache (www-data) is not in group dialout (by default).