I guess this is rather obvious, by why for the constants inside a block. Those constants are going to be used regardless if the file name is 'valid' or not. Example:

if (0) {
   use constant FOO => 5;
}

print FOO, "\n";

__END__
prints 5
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jeffa

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)