You're missing the point of hashes, I'm afraid...

foreach $bkey (sort keys %build_file) {
 
    if (exists $file_list{$bkey})
       print "YES\n";
    }
    else{
       print "NO\n";
    }
}
[download]

The whole idea is that a hash lets you "instantly" look up whether a given key exists in it WITHOUT having to loop over all the keys...

Your code would work, but it would take n*m time, where n's the number of entries in build file, and m's the number in file_list.

This version takes about n time, m times quicker.
--
Mike