Here's all the syntax you need if you don't want to close the file in between read and write, look up the funtions in your perl documentation if need be.

#!perl -w
use strict;

my $filename = "notes.txt";
if (-e $filename) {
  open NOTES, '+< ' . $filename or die "Cant open $filename: $!\n";
}
else {
  open NOTES, '+> ' . $filename or die "Cant open $filename: $!\n";
}
print NOTES "foo\n";
seek NOTES, 0, 0;
$data = <NOTES>;
print $data;
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You may also want to use flock depending on your applications.