use strict;
use warnings;
use DB_File;

tie (my %H, "DB_File", "audit.db", O_RDWR|O_CREAT, 0666) or die $!;
my $doi = '10.1353/ham.2005.0020';

print "|$doi| does ", (exists $H{$doi} ? "exist!\n" : "not exist!\n");

while(my $k = each %H) {
   if ($k eq $doi) {
      print "|$k| eq |$doi|\n";
      last;  # spare me the other millions...
   }
}

untie %H;

##</code><code>##

|10.1353/ham.2005.0020| does not exist!
|10.1353/ham.2005.0020| eq |10.1353/ham.2005.0020|

##</code><code>##

while(my $k = each %H) {
   print "$k does not exist\n"  if (!exists $H{$k});
}