That fails if the caller also uses (say) Parse::RecDescent. If you did want to use %INC, it would be:

use Cwd                   qw( realpath );
use File::Basename        qw( dirname );
use File::Spec::Functions qw( catdir );

my $mod = __PACKAGE__;
$mod =~ s{::}{/}g;
$mod .= '.pm';
$mod = $INC{$mod};

my $module_dir_qfn = dirname(realpath($mod));
my $db_dir_qfn     = catdir($module_dir_qfn, 'db');
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But there's no need to waste four lines getting the file name when it's already provided for you.