my $file = "mydata.txt";

# read data from file
open my $fh, '+<', $file or die "can't open file $file : $!\n";
my @data = <$fh>;
chomp @data;

# modify data (add item)
@data = sort (@data, 'Simon');

# rewind back to beginning
seek($fh, 0, 0);
for my $d (@data) {
  # write data back to file (skips blank lines)
  print $fh "$d\n" if $d;
}
# if new file is shorter than original, throw away extra
truncate($fh, tell($fh));

close $fh;