# Compiling
using File::Spec
using File::Basename
there is a global variable called $some_path
add to path $some_path/lib

_Compilation complete_

# Running
$some_path = dirname(File::Spec->rel2abs(__FILE__));
##</code><code>##

my $some_path;
BEGIN { 
   $some_path = "/my/path";
}
use lib "$some_path/lib"; # Yeah, you don't even need the parenthesis, Perl is clever enough for you
# Edit : thanks dave_the_m for the correction