$ perl-5.14.2 -wE 'say "\xA0" =~ /\s/'
1
$ perl-5.14.2 -wle 'print "\xA0" =~ /\s/u'
1
$ perl-5.12.2 -wE 'say "\xA0" =~ /\s/'

$
[download]

$ perl -wle '$_ = chr 0xA0; utf8::upgrade($_); print /\s/'
1
$
[download]

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