Just to update on some off-forum activity re this thread:
Tux expressed the view that hints/linux.sh should be amended so that -Duse64bitall would create the build I was looking for (ivtype of long, ivsize of 8, ptrsize of 8 and -m64 in ccflags).
But we couldn't get that set correctly. He didn't have access to a powerpc64 system, and my capability with shell scripting is very limited. (More accurately, any such capability is yet non-existent.)

In the midst of this hackery, I found that -m64 needs *also* to be included in cppflags, ldflags and lddlflags.
Eventually I was able to get the build I was after by leaving hints/linux.sh in its original state and running:

sh Configure -des -Dccflags=-m64 -Dldflags=-m64 -Dlddlflags="-m64 -sha
+red -O1" -Dcc=gcc
[download]

That is essentially the same perl as I had been getting with:

sh Configure -des -Dcc="gcc -m64"
[download]

The only difference being that "-m64" was removed from cc, and was inserted into the various flags, as needed - exactly as I was seeking !!
Other than that, the perl -V outputs were identical (with ivtype=long, ivsize=8 and ptrsize=8).

However, it didn't help with the problem that had triggered this investigation.
And, when I think about it again, it's probably not surprising that it did make no difference.
AFAICT, operating in 64-bit mode requires that whenever "gcc" is called, "-m64" must also be called, and having a compiler named "gcc -m64" therefore looks to be a very sane way of ensuring that happens.

Anyway, it sure is good to have the alternative that I was seeking and I'll keep such a build of perl handy for reference.
(But I'm currently planning on staying with the -Dcc="gcc -m64" builds for as long as they don't prove to be deficient.)

Cheers,
Rob