If 0 is assigned to $! before the first stat, the first failed stat does not set the $!.
But it does (as the numerous examples above show). The failed stat fills the Numeric Value of $!, and using $! in your code reads it (Devel::Peek::Dump shows the flags of the var not including pIOK, so the Integer Value is not used). However using -$! forces perl to use the Integer Value instead, which still contains your assigned value.(This was already pointed out in Re^3: Different behaviour of 'stat' function after assignment to $! (errno). by Anonymous Monk, but I thought I might explain it with a bit more words).
This could be considered unwanted behaviour, and you could opt to patch perl to set or clear the IV on error.

On the other hand, you could stop using -$! in your checks :)

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