so a completely portable solution is impossible

That's not really an issue for me. I was just after some way of getting test 6 of PowInt.t to pass on Windows, leaving all other systems unaffected. I think I've now achieved that.

One of the things that was throwing me was that, on Linux, the strings 'nan' and 'inf' are converted to a nan/inf when used in numeric context. That is, on Linux:

perl -MDevel::Peek -e 'Dump "inf" + 1';
[download]

outputs:

SV = NV(0x...) at 0x....
  REFCNT = 1
  FLAGS = (NOK,READONLY,pNOK)
  NV = inf
[download]

But there's no comparable behaviour (that I can find) on Windows. The strings '1.#INF' and '1.#QNAN' are not interpreted as inf/nan when used in numeric context. Instead they're just numified according to the usual rules that apply to the numification of strings - ie they take on a numeric value of 1:

C:\>perl -MDevel::Peek -e "Dump '1.#INF' + 1"
SV = IV(0x218fe28) at 0x218fe2c
  REFCNT = 1
  FLAGS = (PADTMP,IOK,READONLY,pIOK)
  IV = 2
[download]

I wasn't expecting such different behaviour - and that added significantly to my confusion.

Cheers,
Rob