That worked. Your comment, "It sets the special variable to undef", is not yet clear to me. I will have to research this. I greatly appreciate you help. I understand the concept of $_, just not how local $/ modifies it.
Re^2: Unpack Bin file problem
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It doesn't modify the behavior of $_, but of <INFILE> (for any handle).
Perl has a bunch of special variables that change the behavior of perl. $/ Is one of them, it changes what string readline, and thus <INFILE>, searches for as a line terminator, to read a whole line at once. When undef, it'll just make it slurp in the rest of the file.