# Read the file into a hash
#
my %dataFileContains;
open my $fh, 'students.dat' or die "open failed: $!";
my $line;
while  ($line = <$fh>) {
   chomp $line;
   $dataFileContains{$line}++;
}
close $fh;
#
# Read in and test each value, caching those which 
# will be output.
#
my @output;
my ( $name, $age );

while ( ($name, $age ) = getStudentData() ) {
    my $outLine = "$name\t$age";
    push @output, $outLine unless $dataFileContains{$outLine};
}
#
# Output the data.
#
if ( scalar @output ) {
    open my $fh, '>>students.dat' or die "open failed: $!";
    foreach ( @output ) {
       print $fh, "$output\n";
    }
}