$BUFF is well-declared, so why would it stop being that all of a sudden?

It's still "well-declared".
The warning is not that it has not been declared, but that it has not been assigned a value:

C:\>perl -wle "my $x; print $x;"
Use of uninitialized value $x in print at -e line 1.

C:\>perl -wle "my $x; $x = 17; print $x;"
17
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Cheers,
Rob