Why would prepending  "AUTH PLAIN " to the encoded string leave the encoding unchanged so that decoding would remain symmetric?

c:\@Work\Perl\monks>perl -wMstrict -le
"use MIME::Base64;
 ;;
 use Data::Dump qw(pp);
 ;;
 my $encoded = encode_base64('Aladdin:open sesame');
 ;;
 my $decoded = decode_base64($encoded);
 print qq{A: '$decoded'};
 ;;
 $encoded = 'AUTH PLAIN ' . $encoded;
 $decoded = decode_base64($encoded);
 print 'B: ', pp $decoded;
"
A: 'Aladdin:open sesame'
B: pack("H*","0144c73cb0083505b1859191a5b8e9bdc195b881cd95cd85b594")
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Update: And yes, this does look like a bit of a new topic...