#!/usr/bin/perl
use strict;
use warnings;

open my $fh, '<', 'foo.txt' or die $!;
seek($fh, 0, 2);
print "File size: ", tell($fh), "\n";
print "Change file and press enter ....\n";
<STDIN>; # jetzt editiere die Datei
seek($fh, 0, 2);
print "New file size: ", tell($fh), "\n\n";
seek($fh, 0, 0);
print <$fh>;
close $fh;