How do I decode a MIME/BASE64 string?

The MIME-tools package (available from CPAN) handles this and a lot more. Decoding BASE64 becomes as simple as: 

    use MIME::base64;
    $decoded = decode_base64($encoded);

A more direct approach is to use the unpack function's ``u'' format after minor transliterations: 

    tr#A-Za-z0-9+/##cd;                   # remove non-base64 chars
    tr#A-Za-z0-9+/# -_#;                  # convert to uuencoded format
    $len = pack("c", 32 + 0.75*length);   # compute length byte
    print unpack("u", $len . $_);         # uudecode and print