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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