\s is suppose to match U+00A0 and sometimes does. It's a bug in Perl that cannot be fixed for backwards compatibility reasons. You can indicate you want the fix using feature unicode_strings. In earlier versions of Perl, you can also work around the bug by upgrading the string's internal format.
use strict;
use warnings;
use feature qw( say );
my $string = chr(160);
no feature qw( unicode_strings );
say $string =~ /^\s*$/ ? 1 : 0 ; # 0
use feature qw( unicode_strings );
say $string =~ /^\s*$/ ? 1 : 0; # 1
no feature qw( unicode_strings );
utf8::upgrade($string);
say $string =~ /^\s*$/ ? 1 : 0; # 1
