Sorry for my poor English.
After previous post I've thought once again and now I think than intuitively $2=undefined should be incorrect, and $2=a correct.

After that I've received an email from guru of regex Jeffrey Friedl (regex.info):
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Hi Serge,
I've been thinking about this for a while, and as far as I can tell it does seem
to be a bug. By definition, $2 must be defined before the (?{...}) can run.
It's probably a problem with how it backtracks. I'd suggest filing a bug report..

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Splitting the regex:
((\w+)(?{print...}))((\w+)(?{print...}))
is wrong, really the regex is not split.
After (\w+) captures all the string:
(\w+)) | {2}
we see, that second repetition of \w not match. We do backtracking and enter second parentheses going from right to left:
(/w | )+
In this case the regex engine (as I think) set $2=undefined, but why? Intuitively it seems set $2=undefined should do after we leave the open second parenthesis going from right to left.