It seems the \\., which captures the \" in the target, is not possessive, and is causing the error.

The following works in the given case. Are there cases where it fails?

print "NOT MATCHED!\n" unless
  /^ " (?: [^"\\]++ | (?: \\. )++ )*+ " /x ;
[download]

Update:

Playing around more, it seems that the possessives are not needed on the internals, but only that the \\. should have a quantifier:

print "NOT MATCHED!\n" unless
  /^ " (?: [^"\\]+ | (?: \\. )+ )*+ " /x ;
[download]

This works on a string of length 100 million, in <2s on my machine. (I didn't try any longer strings.)

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