doubt about $ in perldoc perlvar

[Atul_Khot]

Gurus,
        perldoc perlvar says:

     $<digits>
             Contains the subpattern from the corresponding set
             of parentheses in the last pattern matched, not
             counting patterns matched in nested blocks that have
             been exited already.  (Mnemonic: like \digits.)
             These variables are all read-only.

I don't understand the second part ==> "not counting patterns matched in 
nested blocks that have been exited already." Could I get some example 
illustrating this point?

-- Thanks,
Atul