I tried it with if ! instead of unless. Worked. 
package Chess::PGN::EPD;

use strict;
use warnings;
use Chess::PGN::Moves;
use File::Spec::Functions;
use Storable qw( retrieve );
use Cwd qw( realpath );
use Try::Tiny qw( try catch );

require Exporter;

my ( $hECO, $hNIC, $hOpening );
my (%hash) = (
    ECO     => \$hECO,
    NIC     => \$hNIC,
    Opening => \$hOpening
);

my $db_dir_qfn = realpath( catfile( __PACKAGE__, updir(), 'db' ));
if ( -d !$db_dir_qfn  ) {
    $db_dir_qfn = realpath('db');
}

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In reply to Re: Finding files relative to a module by Khen1950fx
in thread Finding files relative to a module by hsmyers

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