I have learned today that you could force the regex engine to fail and backtrack, for example with this: 
"ABCDEF" =~ /([A-Z]{3})(?{print "$1\n"})(?!)/;

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trizen explained to me how this regex worked and this is what I understood:

First it matches ABC, then prints it, then fail, then backtracks to the position where the last match occured + 1.

Then when it runs again, it will match BCD this time. Thus this loop prints: 

ABC
BCD
CDE
DEF

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However based on this reasoning, I struggle to understand the next example: 

"ABCDEF" =~ /(\w{2,}?)(?{print "$1\n"})(?!)/;

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Here it matches AB first then prints it, then fails then backtracks. But here, instead of matching BC as I would expect it, it matches ABC. So after failing, it does not backtrack to pos+1 like in the previous example. And the loop prints: 
AB
ABC
ABCD
ABCDE
ABCDEF
BC
BCD
BCDE
BCDEF
CD
CDE
CDEF
DE
DEF
EF

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Can someone explain me what is wrong in my assumption?

In reply to regex backtracking question by aeqr

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