Or you could do it conditionally. Prints either 2 B's or something that's not 2 B's.

#!/usr/local/bin/perl -l
 
foreach ('b', 'bb', 'B', 'foo', 'bar', 'BB') {
    print m/^b{2}$/i ? "\x42\x42" : bb(); 
}
sub bb() {
    while(1) { return chr(rand(100)) . chr(rand(100)) unless (m/^b{2}$
+/i); }
}
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