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.

In reply to Re^3: Unpack Bin file problem by bart
in thread Unpack Bin file problem by mark4444az

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