use Encode qw( encode );

my $s = do { use utf8; "高" };
print(length($s), "\n");   # 1

open my $fh, '>:utf8', \$s2;
print $fh $s;
print(length($s2), "\n");  # 3

$s2 = encode('utf8', $s);
print(length($s2), "\n");  # 3