[tachyon@www cgi-bin]# cat ./test.pl
#!/usr/bin/perl

my $file = '/tmp/test.txt';

open FILE, ">$file" or die $!;
print FILE "foo\nbar\nfoo\nbar\nfoo\nbar\nfoo\nbar\nfoo\nbar\nfoo\nbar\n\n\r\n\n\r";
close FILE;

# grab the last 20 bytes for analysis
open FILE, $file or die $!;
{
  local $/ = undef;
  seek FILE, -20, 2;
  $EOF = <FILE>;
}
close FILE;

my $file_length = -s $file;
(my $newlines) = $EOF =~ m/([\015\012]+)\z/;
my $num_newlines = length $newlines;
print "File length $file_length bytes, number of newlines $num_newlines\n";

truncate $file, ($file_length - $num_newlines) or die $!;
print "Now file is ", -s $file, "bytes\n\n";

[tachyon@www cgi-bin]# ./test.pl
File length 53 bytes, number of newlines 6
Now file is 47 bytes

[tachyon@www cgi-bin]#