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