I didn't understand either, and I'm lazy so:

use YAPE::Regex::Explain;
my $regex=YAPE::Regex::Explain->new(qr/((.))/);
print $regex->explain;
__END__
The regular expression:

(?-imsx:((.)))

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    (                        group and capture to \2:
----------------------------------------------------------------------
      .                        any character except \n
----------------------------------------------------------------------
    )                        end of \2
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
[download]

so your foreach makes this is evaluated in list-context and \1 and \2 are passed.