sub countBytes {
 my $s=$_[0];   #makes copy
 Encode::_utf8_off($s);
 return length($s);
}

# or to save memory by avoiding a copy for loooong strings
# BUT note: may not be a good idea if string is shared by multiple
# threads since this is not atomic and another thread could grab
# control while the utf8 bit is temporarily off.
# The copy approach is more stable and thread friendly.

sub countBytes {
  my $bUtf = Encode::is_utf8($_[0]);
  Encode::_utf8_off($_[0]);
  my $i=length($_[0]);
  Encode::_utf8_on($_[0]) if $bUtf;
  return $i;
}

#calc bytes before printf to show flag is indeed preserved
my $s=chr(0x160);
my $iBytes = countBytes($s);

printf "chr=<%s> utf8-flag=%s length=%d bytes=%s\n"
  , $s, Encode::is_utf8($s)?'yes':'no', length($s), $iBytes;
#outputs: chr=<?> utf8-flag=yes length=1 bytes=2