Encode::MIME::Header -- MIME 'B' and 'Q' header encoding


    use Encode qw/encode decode/;
    $utf8   = decode('MIME-Header', $header);
    $header = encode('MIME-Header', $utf8);


This module implements RFC 2047 Mime Header Encoding. There are 3 variant encoding names; MIME-Header, MIME-B and MIME-Q. The difference is described below

              decode()          encode()
  MIME-Header Both B and Q      =?UTF-8?B?....?=
  MIME-B      B only; Q croaks  =?UTF-8?B?....?=
  MIME-Q      Q only; B croaks  =?UTF-8?Q?....?=


When you decode(=?encoding?X?ENCODED WORD?=), ENCODED WORD is extracted and decoded for X encoding (B for Base64, Q for Quoted-Printable). Then the decoded chunk is fed to decode(encoding). So long as encoding is supported by Encode, any source encoding is fine.

When you encode, it just encodes UTF-8 string with X encoding then quoted with =?UTF-8?X?....?= . The parts that RFC 2047 forbids to encode are left as is and long lines are folded within 76 bytes per line.


Before version 2.83 this module had broken both decoder and encoder. Encoder inserted additional spaces, incorrectly encoded input data and produced invalid MIME strings. Decoder lot of times discarded white space characters, incorrectly interpreted data or decoded Base64 string as Quoted-Printable.

As of version 2.83 encoder should be fully compliant of RFC 2047. Due to bugs in previous versions of encoder, decoder is by default in less strict compatible mode. It should be able to decode strings encoded by pre 2.83 version of this module. But this default mode is not correct according to RFC 2047.

In default mode decoder try to decode every substring which looks like MIME encoded data. So it means that MIME data does not need to be separated by white space. To enforce correct strict mode, set package variable $Encode::MIME::Header::STRICT_DECODE to 1, e.g. by localizing:

require Encode::MIME::Header; local $Encode::MIME::Header::STRICT_DECODE = 1;

It would be nice to support encoding to non-UTF8, such as =?ISO-2022-JP? and =?ISO-8859-1?= but that makes the implementation too complicated. These days major mail agents all support =?UTF-8? so I think it is just good enough.

Due to popular demand, 'MIME-Header-ISO_2022_JP' was introduced by Makamaka. Thre are still too many MUAs especially cellular phone handsets which does not grok UTF-8.



RFC 2047, and many other locations.