1) Instead of changing the #! line, you could have OS-specific .pl's which initialize variables to OS-specific values, the uses do to execute the common code.

2) You shouldn't need to use $sep. For that, we have core modules File::Spec and File::Basename.

applic_linux.pl
---------------
#!/usr/bin/perl

our $logdir = "/home/ghodmode/logs";

do 'applic.pl';


applic_win32.pl
---------------
#!c:\perl\bin\perl

our $logdir = "c:\\xampp\\xampp\\webapp\\logs";

do 'applic.pl';


applic.pl
---------
use strict;
use warnings;

BEGIN {
   our $logdir;
   die(...) unless defined $logdir;
}

use File::Spec ();

my $logfile = File::Spec->catfile($logdir, 'error_log.txt');

...
[download]

Better yet would be to use a config file to hold the name of the log dir. That would be both OS-independant and more useful.