Short explanation: (?{...}) means to execute arbitrary Perl code inside a regular expression, and (?!) makes the regex engine to fail and backtrack, trying to match from the last_pos + 1. When it starts matching ABC, it prints it, fails, backtracks and starts matching from B the next three letters, giving us BCD. The process repeats until the internal regex counter reaches the end of the string.

I know, I'm really bad at explaining things to humans, but, fortunately, Athanasius explained this better once.

Please see: Re: RegEx + vs. {1,}