DIN is a file handle. The syntax is actually depreciated. The more correct way is this:

use strict; # always use this for better code.
use warnings; # always use this for more meaningful error messages.
use English;

my $din;

open( $din, "<", $filename )
   or die "Cannot open file, $filename, error: $ERRNO";
while (<$din>) # process file one line at a time
{
   # do stuff hear on each line.
}
close( $din );
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