Welcome to the Monastery and the wider Perl world, oldB51. You've made a great start already.

It is likely that my debug tactic will be to print variables at various stages until the problem is found. This is usually easier than a formal debugger anyway.

I too am an advocate of liberal print/warn statements while debugging. However you may be unaware that Perl has its own built-in debugger which can you use at any time should you so wish. It is explained extensively in perldebug. See also the FAQ How do I debug my Perl programs?

I think I’m right in saying neither will in fact install on 64bit Macs. But - if this is the case - why does the installation begin. Surely cpanm knows what system it is trying to install into and should stop the process immediately with a polite message.

It does know the architecture but it doesn't necessarily know that any given module won't install there. That is in the gift of the module author and they may not know either. Many modules were originally written before 64-bit architectures were widely available, for example, and some have not kept pace with such developments. It might also be the case that it's just a bug and needs to be fixed.

Contrarily many modules will bail out early if it is attempted to install them on a platform which is known to be unsuitable. This is most often seen when a non-Perl pre-requisite is missing (a compiler, a system libarary, a third-party tool, etc.). You will probably come across one of these eventually.