I don't think that it's Cygwin - or at least not only Cygwin. Here are my results on OS X. The 5.8 is the system Perl, and the 5.10 is my own:

hektor ~ $ ./retest 
Perl 5.10.0 on darwin:
$ARGV = {
          'WHEN' => undef
        };
$string = 'It is thyme';

hektor ~ $ /usr/bin/perl retest 
Perl 5.8.8 on darwin:
$ARGV = {
          'WHEN' => '12:34'
        };
$string = 'It is thyme';

[download]

A quick second look: I'm now thinking it has something to do with the configuration. These two are from the same machine (Debian Lenny), but they're both 5.10.0. Nevertheless, the results differ. The first 5.10.0 is once again my own build, but the second is Debian's. (I applied a few patches to my own, particularly one to stop a regular expression related memory leak, but I believe that Debian has that patch as well - and more.)

telemachus ~ $ perl testre 
Perl 5.10.0 on linux:
$ARGV = {
          'WHEN' => undef
        };
$string = 'It is thyme';

telemachus ~ $ /usr/bin/perl testre 
Perl 5.10.0 on linux:
$ARGV = {
          'WHEN' => '12:34'
        };
$string = 'It is thyme';

[download]

In reply to Re^2: Getopt::Euclid victim of 5.10 upgrade by telemachus
in thread Getopt::Euclid victim of 5.10 upgrade by scott\b

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