truncate('test.dat', 1024);

##</code><code>##

use strict;
use warnings;

my $fname = 'test.dat';
my $crop = 1024;

my $handle;
open($handle, $fname);
$_ = (stat($handle))[7];
if ($_ > $crop) {
    seek($handle, $_ - $crop, 0);
    read($handle, $_, $crop);
    close($handle);
    open($handle, ">$fname");
    print $handle $_;
}
close($handle);

##</code><code>##

my $crop = 1024;
my $margin = 64;
...
read($handle, $_, $crop + $margin);