What this is doing is converting Base64 to uuencoded format. They both encode 6 bits per character, but use different characters -- Base64 uses 'A' to encode 0, where uuencode uses ' ' or "`"; Base64 uses 'B' for 1, uuencode uses '!'; and so on, as per the  tr|A-Za-z0-9+/| -_| you quote.

You'll see that the uuencoded characters are decoded elsewhere.