Take a look at Regexp::Debugger. I've been using this module for about a decade and can highly recommend it.

"How to surely determine whichever Perl regex group fail ..."

It doesn't really work like that. Assuming you've installed Regexp::Debugger, you can run

$ rxrx -e 'q{okoK} =~ /(o|k)(k|i)(O|K)(K)/'
[download]

and watch:

1. 1st group (o|k) match success on 1st alternative ($1 = "o")
2. 2nd group (k|i) match success on 1st alternative ($2 = "k")
3. 3rd group (O|K) match fail
4. BACKTRACK
5. 2nd group (k|i) match fail on 2nd alternative ($2 removed)
6. BACKTRACK
7. 1st group (o|k) match fail on 2nd alternative ($1 removed)
8. ... Regex failed to match after 66 steps ...

So, as you can see, capture groups can potentially both succeed and fail at different points in the matching process.