sub _quote_envid {
  local $_ = shift;

  # RFC 3461: max 100 chars, pass through printable ASCII characters excluding
  # [ =+], encode others to '+XX' as uc(hex(ord($char)))
  s{([^\x21-\x7e]|[=+]))}{sprintf "+%02X", ord($1)}ge;

  # we're usually encoding an email address, so trim long strings from front
  # to preserve the domain
  s/^(\+..|.)// while 99 < length;

  $_;
}