The bit on the end indicates the most recent line of the most recent file read. In the case shown, the main script probably has lines that looks something like:

open GEN1, ...
while( <GEN1> ) {
 ...
}
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From perlfunc for die:

If the last element of LIST does not end in a newline, the current script line number and input line number (if any) are also printed, and a newline is supplied. Note that the ``input line number'' (also known as ``chunk'') is subject to whatever notion of ``line'' happens to be currently in effect, and is also available as the special variable $..

It's a barely documented and somewhat dubious feature.