Thanks to those that replied.

Jan "jdb" Dubois on #win32@irc.perl.org suggested a fix that works well.

use Time::Piece;

$ENV{TZ} = 'UTC';
Time::Piece::_tzset();
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According to Jan, this requires atleast Time::Piece 1.14 and is fully fixed in 1.17.

There is also a tzset() in POSIX which I had initially considered but it gives a "not implemented" error on current strawberry perl. Hopefully that is just a bug which will soon be rectified.

Update: Time::Piece 1.12 appears to work as well.