Done as suggested and included the printing of a header ... still doesn't work, but the symptoms have now changed (no surprises there then, TFT M$). The error is now a 404 error - the so-called error log indicates an alternating sub-status of 00 and 02 !?!

• The file is most definitely present (at the browsed location),
• A web service extension is enabled allowing access to the perl binary and ...
• that self-same binary is configured as being required to run files having a '.pl. extension and moreover, ...
• The indicated perl binary has identical permissions to a DLL that can be run from the same location.

P.S. I have downloaded the IIS 6 toolkit - all the tools contained therein appear to do is to present the 404 error web page in a number of different formats. What an idiot for thinking it might be useful and give me a clue (no matter how small) as to WTF is going on !!