SELECT FROM mytable WHERE hashkey = '4ea' && path = '/my/path/to/file/isthis.dat';

##</code><code>##

use Digest::MD5 qw(md5 md5_hex md5_base64);

my $path = '/my/path/to/file/isthis.dat';
my $hashkey = substr(md5_hex($path), 0, 3);