use FindBin;
use lib "$FindBin::Bin/../lib";

or:

use FindBin qw($Bin);
use lib "$Bin/../lib";

FindBin exports the following variables:

$Bin

Path to bin directory from which script was invoked

$Script

Basename of script from which Perl was invoked

$RealBin

$Bin with all links resolved

$RealScript

$Script with all links resolved

If you invoke Perl with the -e option or read the Perl script from STDIN, then FindBin sets both $Bin and $RealBin to the current directory.

8.114. Filter::Util::Call		8.116. GDBM_File