... I found '\G' is usable, but ... it returns an extra pair of matches that contain undef ...

This has nothing to do with the  \G assertion, but is a facet of the way unmatched capture groups behave in list context when allowed to match zero times. Consider:

c:\@Work\Perl>perl -wMstrict -le
"use Data::Dump qw(pp);
 ;;
 my $qr_string = q((?:\G(\w)\W{2}(\w))*);
 my $qr = qr[$qr_string];
 ;;
 for my $s ('', qw(a--b  c--dE--F  g--hI--Jk--l)) {
   my @caps = $s =~ /$qr/g;
   print qq{'$s' -> }, pp \@caps;
   }
"
'' -> [undef, undef]
'a--b' -> ["a", "b", undef, undef]
'c--dE--F' -> ["c", "d", "E", "F", undef, undef]
'g--hI--Jk--l' -> ["g", "h", "I", "J", "k", "l", undef, undef]

c:\@Work\Perl>perl -wMstrict -le
"use Data::Dump qw(pp);
 ;;
 my $qr_string = q((?:(\w)\W{2}(\w))*);
 my $qr = qr[$qr_string];
 ;;
 for my $s ('', qw(a--b  c--dE--F  g--hI--Jk--l)) {
   my @caps = $s =~ /$qr/g;
   print qq{'$s' -> }, pp \@caps;
   }
"
'' -> [undef, undef]
'a--b' -> ["a", "b", undef, undef]
'c--dE--F' -> ["E", "F", undef, undef]
'g--hI--Jk--l' -> ["k", "l", undef, undef]
[download]

Both of the variations above, with and without the  \G assertion
    q((?:\G(\w)\W{2}(\w))*)
and
    q((?:(\w)\W{2}(\w))*)
but with a  * quantifier on the  (?:...) group containing the capture groups, produce pairs of spurious undef values, although the other values generated are different. Versions of the regex eliminating the  * quantifier (or using a  + quantifier, but no example of this is given) do not produce spurious undefs:

c:\@Work\Perl>perl -wMstrict -le
"use Data::Dump qw(pp);
 ;;
 my $qr_string = q((?:\G(\w)\W{2}(\w)));
 my $qr = qr[$qr_string];
 ;;
 for my $s ('', qw(a--b  c--dE--F  g--hI--Jk--l)) {
   my @caps = $s =~ /$qr/g;
   print qq{'$s' -> }, pp \@caps;
   }
"
'' -> []
'a--b' -> ["a", "b"]
'c--dE--F' -> ["c", "d", "E", "F"]
'g--hI--Jk--l' -> ["g", "h", "I", "J", "k", "l"]

c:\@Work\Perl>perl -wMstrict -le
"use Data::Dump qw(pp);
 ;;
 my $qr_string = q((?:(\w)\W{2}(\w)));
 my $qr = qr[$qr_string];
 ;;
 for my $s ('', qw(a--b  c--dE--F  g--hI--Jk--l)) {
   my @caps = $s =~ /$qr/g;
   print qq{'$s' -> }, pp \@caps;
   }
"
'' -> []
'a--b' -> ["a", "b"]
'c--dE--F' -> ["c", "d", "E", "F"]
'g--hI--Jk--l' -> ["g", "h", "I", "J", "k", "l"]
[download]

Don'cha just love regexes? Play with variations of these patterns (including  qr[$qr_string*] and  qr[$qr_string+]) for deeper confu... um, greater enlightenment.

So what's going on? Here's how I would describe it: If the  (?:...(...)...(...)) group containing two capture groups is allowed to match zero times at some point, e.g., the end of the string, it will! However, the capture groups inside it don't actually capture anything, so they return undef.

Compare that behavior to unmatched capture groups in an alternation:

c:\@Work\Perl>perl -wMstrict -le
"use Data::Dump qw(dd);
 ;;
 my $s = 'aBcDeFg';
 ;;
 my @captures = $s =~ m{ (B) | (D) | (F) }xmsg;
 dd \@captures;
"
["B", undef, undef, undef, "D", undef, undef, undef, "F"]
[download]

Also consider:

c:\@Work\Perl>perl -wMstrict -le
"use Data::Dump qw(dd);
 ;;
 my $qr_string = q((?:(\w)\W{2}(\w))*);
 my $qr = qr[$qr_string+];
 ;;
 my $s = '%%%%';
 print 'MATCH!!!' if $s =~ /$qr/g;
 dd \@-;
 ;;
 my @captures = $s =~ /$qr/g;
 dd \@captures;
"
MATCH!!!
[0]

[undef, undef, undef, undef, undef, undef, undef, undef]
[download]

Update: In place of the last example, consider instead:

c:\@Work\Perl>perl -wMstrict -le
"use Data::Dump qw(pp);
 ;;
 my $qr_string = q((?:(\w)\W{2}(\w))*);
 my $qr = qr[$qr_string];
 ;;
 my $s = '%%%%';
 ;;
 print 'match @ offset ', $-[0], '  ($1, $2)==', pp $1, $2  while $s =
+~ /$qr/g;
 ;;
 my @captures = $s =~ /$qr/g;
 pp \@captures;
"
match @ offset 0  ($1, $2)==(undef, undef)
match @ offset 1  ($1, $2)==(undef, undef)
match @ offset 2  ($1, $2)==(undef, undef)
match @ offset 3  ($1, $2)==(undef, undef)
match @ offset 4  ($1, $2)==(undef, undef)
[undef, undef, undef, undef, undef, undef, undef, undef, undef, undef]
[download]

For discussion of $-[0], please see @- in perlvar. Also note that the definition
    my $qr = qr[$qr_string];
was changed from the previous example to remove the  + quantifier, which was included accidentally and only served to obscure the example.

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