#!/bin/perl -w
use strict;

open( NOTES, "+<note.txt")             # +< allows you to do read/write updates on the file
  or die "cannot open note.txt: $!";

print NOTES "stuff\n";                 # print stuff in your file

my $pos= tell NOTES;                   # mark the position before the write you are interested in
print NOTES "test\n";                  # write your data (it should end up with \n if you want 
                                       # <NOTES> to pick it up properly later)

# when you are ready to read it

seek NOTES, $pos, 0;                   # reset filehandle to the right position
my $fromfile=<NOTES>;                  # read
chomp $fromfile;                       # rememebr that \n? You might want to remove it now
if ($fromfile eq "test") {print "Yes, this works"}