Hi, replying to myself for the archives. Thanks to Alexandr Ciornii (chorny) of CPAN testers, I've narrowed this down to an apparent bug in Strawberry Perl 5.14.3 .
I don't have a Win32 machine myself but chorny was kind enough to test a minimal script with only:
use open ':std', ':encoding(utf8)';
and it threw the same error. Removing the 'use open' line fixed the problem as syphilis guessed.
I installed 5.14.3 on my OS X machine and the test passes fine so it appears to be that particular version of Strawberry Perl.
Not going to file a bug report because I can't provide any useful diagnostic info. If anyone with Strawberry Perl 5.14.3 wished to do so, the gods would smile upon her or him.
The way forward always starts with a minimal test.