This suggests it's not COW related?

Well, it's certainly not related to the introduction of the new COW in perl-5.20.0.
I've just fired up my old windows 7 machine that has all of the ".0" stable releases from 5.10.0.
The demo script ran fine from 5.18 to 5.26.0 - I didn't test back any further back than 5.18.0.
The slowdown comes with perl-5.28.0, and then persists right up to (and including) perl-5.36.0 - which is the latest perl version on that machine.
A quick browse of the perl5280delta didn't really sound any alarms, though I noticed the following:

  [CVE-2017-12814] $ENV{$key} stack buffer overflow on Windows
    A possible stack buffer overflow in the %ENV code on Windows has b
+een
    fixed by removing the buffer completely since it was superfluous a
+nyway.
    [GH #16051] <https://github.com/Perl/perl5/issues/16051>
[download]

Is there possibly anything suspicious in that ? (It was the fact that it involves the removal of a buffer that led me to wonder ...)

Cheers,
Rob