Note that the feature unicode_strings was introduced in 5.12, but to make \s match \xA0 on a non-utf-8 strings, you need 5.14:

$ perl-5.14.2 -wE '$_ = chr 0xA0; say /\s/ || 0'
1
$ perl-5.12.2 -wE '$_ = chr 0xA0; say /\s/ || 0'
0
[download]

The -E enables "use feature 'unicode_strings'".