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

use strict;

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

open my $fh, '<', 'students.dat' or die "open failed: $!";
open my $tmp, '>', 'students.tmp' or die "open failed: $!";

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

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

close $fh;
close $tmp;
rename 'students.tmp', 'students.dat';