#!/usr/bin/perl -w
use strict;

die "Usage: $0 file numlines\n" unless @ARGV == 2;

my ($filename, $numlines) = @ARGV;

# open or die first so that we don't stat a file we don't know exists
open my $fh, "<", $filename or die "Couldn't open $filename: $!";

my $chunksize = 2 * 200 * $numlines;
my $filesize = -s $fh;
$chunksize = $filesize if $filesize < $chunksize;

# why seek twice?
seek $fh, -$chunksize, 2;

my @tail = <$fh>;
splice @tail, 0, @tail - $numlines;
print @tail;

__END__
$ perl -le'print "a"x500 for 1..10' > t.dat
$ tailz t.dat 5 | wc -l
      1