I tried "m//" as reported on your 'next' expression and got 'd e f g h', as expected. From my perlop on 5.36.

    The empty pattern "//"
            If the *PATTERN* evaluates to the empty string, the last
            *successfully* matched regular expression is used instead. In
            this case, only the "g" and "c" flags on the empty pattern are
            honored; the other flags are taken from the original pattern. If
            no match has previously succeeded, this will (silently) act
            instead as a genuine empty pattern (which will always match).

            Note that it's possible to confuse Perl into thinking "//" (the
            empty regex) is really "//" (the defined-or operator). Perl is
            usually pretty good about this, but some pathological cases
            might trigger this, such as "$x///" (is that "($x) / (//)" or
            "$x // /"?) and "print $fh //" ("print $fh(//" or
            "print($fh //"?). In all of these examples, Perl will assume you
            meant defined-or. If you meant the empty regex, just use
            parentheses or spaces to disambiguate, or even prefix the empty
            regex with an "m" (so "//" becomes "m//").