Resolution:
When using PDriver with an earlier version of LiveCluster, implicit, comma-seperated arrays could be created from the output of a command as follows:

implicitArray = `cat $file`

As of GridServer 3.2, PDriver now has explicit support for array types, which differs from the 3.1.1 syntax and functionality. The reason for this being that CSV lists tended to be error-prone and that users did not always want the output of grave-marked commands to be automatically converted to CSV format.

Therefore, in order to convert the output of `type $inputfile` to an array type upon which sizeof can operate, the output of the grave-marked command should be passed through the split function, which will convert it. Therefore, the script should be changed as follows:

explicitArray = split(`type $inputfilename`)

Further details may be found in the PDriver documentation, and the example PDS script new_syntax.pds.

PDriver implicit arrays don't work in GridServer 3.2 and later