use constant is_perl55 => ($] < 5.005_50);
use constant is_perl56 => ($] < 5.007 && $] > 5.005_50);
use constant is_sane_perl => $] > 5.007;
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Weird. The "fat comma" ("=>") quotes its left hand side, so strict shouldn't complain. Looks like your perl is broken. Did your Win10 do any updates lately? Carefully check C:/Perl64/lib/ExtUtils/Constant/Utils.pm for any weirdness, and you could move the original file out of the way, make a copy and quote the LHS of the arguments to use, e.g.

use constant 'is_perl55' => ($] < 5.005_50);
use constant 'is_perl56' => ($] < 5.007 && $] > 5.005_50);
use constant 'is_sane_perl' => $] > 5.007;
[download]

If that fixes the issue, it is time to file a bug report to ActiveState.