$c //= $d;
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is just syntactic sugar for various constructs such as:

$c = defined $c ? $c : $d;
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or

$c = $d unless defined $c;
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Update: I've just quickly tried on an old VMS platform with Perl 5.8.6 with this regex definition:

 $regex = qr# (\d) (??{ my $c = $1 unless defined $c; ++$c })+ #x;
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it still does not seem to work.