You refer to a .pl-file. This is not really a program-file, but rather a library of multiple files. With the prolib command you can extract or list of the contents of the lib.
The content are mostly compiled (.r) files, which can be compiled with met min-size option. In that case it is impossible to debug.
In the other case (you can always try) you must find out which program is beeing called into the .pl (perhaps you can find out while editing the .pf) and change this call to your procedure where you intialize the debugger (debug command)
