I should have taken tilly's suggestion from the VERY start and used Temp::File. And now I feel quite foolish, but perhaps a little bit wiser, and I must acknowledge my folly to the Perlmonks who've endured me.

Especially since the solution which would solve this mystery was in none of the pieces of code I posted here. I realized that the original block of code I posted in the parent thread was in a subroutine which had been called by ANOTHER subroutine, and in THAT subroutine I had already opened and flocked a tempfile which had been created moments ago with the exact same naming scheme.

So the most obvious solution was invisible to me late at night: "The file exists, and that's why you can't get a lock on it, dumbass!"
Thankfully things were clearer in the morning. I feel sooo silly.