Is this a bug?

The regexp debugger shows that in the /g case, the last match in the "implicit loop" of /g is a failed one (see also Repeated Patterns Matching a Zero length Substring), probably leading to that result in @-; note that $+ and $^N are also undef in this case. In addition to the below, try perl -MRegexp::Debugger -e '$_="a";s/(?=(a))/b/g'.

However, using those variables after a /g match feels a little sketchy to me, so this may be an XY problem, so if you could explain the bigger picture, perhaps we can help solve that.

$ perl -Mre=debug -e '$_="a";s/(?=(a))/b/g'
Compiling REx "(?=(a))"
Final program:
   1: IFMATCH[0] (11)
   3:   OPEN1 (5)
   5:     EXACT <a> (7)
   7:   CLOSE1 (9)
   9:   SUCCEED (0)
  10: TAIL (11)
  11: END (0)
minlen 0 
Matching REx "(?=(a))" against "a"
   0 <> <a>                  |   0| 1:IFMATCH[0](11)
   0 <> <a>                  |   1|  3:OPEN1(5)
   0 <> <a>                  |   1|  5:EXACT <a>(7)
   1 <a> <>                  |   1|  7:CLOSE1(9)
   1 <a> <>                  |   1|  9:SUCCEED(0)
                             |   1|  SUCCEED: subpattern success...
   0 <> <a>                  |   0| 11:END(0)
Match successful!
Matching REx "(?=(a))" against "a"
   0 <> <a>                  |   0| 1:IFMATCH[0](11)
   0 <> <a>                  |   1|  3:OPEN1(5)
   0 <> <a>                  |   1|  5:EXACT <a>(7)
   1 <a> <>                  |   1|  7:CLOSE1(9)
   1 <a> <>                  |   1|  9:SUCCEED(0)
                             |   1|  SUCCEED: subpattern success...
   0 <> <a>                  |   0| 11:END(0)
END: Match possible, but length=0 is smaller than requested=1, failing
+!
   1 <a> <>                  |   0| 1:IFMATCH[0](11)
   1 <a> <>                  |   1|  3:OPEN1(5)
   1 <a> <>                  |   1|  5:EXACT <a>(7)
                             |   1|  failed...
                             |   0| failed...
Match failed
Freeing REx: "(?=(a))"
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