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');

...