Sometimes I just love doing homework :) If you plan on using this for a class, you'd better make sure you understand everything I've used. You wouldn't want a prof to ask "what does _that_ do?" and you end up blubbering a "I don't know what _that_ does".

#!c:/perl/bin/perl -w
$|++;

use strict;
use Fcntl ':flock';

print "Enter new student's name: ";
chomp( my $student = <STDIN> );

open my $fh, '+<', 'students.dat' or die "open failed: $!";
flock $fh, LOCK_EX or die "flock failed: $!";

my $found = 0;
while (<$fh>) {
   $found = 1 if /^\Q$student\E$/i;
}

print $fh $student, "\n" unless $found;

close $fh;
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