/((?:(?:[^\n@]+|@[^@]*@)\n?)+)/gs

Just a bit of optimization: I think the inner noncapturing parens would better be replaced with a "cut" (non-backtracking) pattern. You have nested plusses, and that could occasionally go very awry. See Jeffrey Friedl's book on regular expressions for a detailed discussion.

Oh, as there are no dots in the pattern, the /s modifier is useless.

/((?:(?>[^\n@]+|\@[^@]*\@)\n?)+)/g
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There. That should match the same things, but without danger for near-endless backtracking.

Never mind the warning in perlre, I see no reason why this particular feature would have to change as it has been in use for many years, and unlike other extensions like (?{ CODE }) and (??{ EXPR }), it is very simple, and effective. For the latter extensions, I think they'll stay too, although the API may still change. (They are not simple.)