What is the definition of your sub _compare?
(incomplete code cannot possibly demonstrate the problem)
Use the find_dup method to see if there are any duplicates.
(change the format of your values to help you out here, or just forget the whole R_DUP thing, and do something with the keys like some_key_part_1, some_key_part_2 ...)
Here is an example adapted from the DB_File documentation to show you a neat little trick
use strict ;
use DB_File ;
use vars qw( $x %h ) ;
# Enable duplicate records
$DB_BTREE->{'flags'} = R_DUP ;
$x = tie %h, "DB_File", undef, O_RDWR|O_CREAT, 0640, $DB_BTREE
or die "Cannot open $filename: $!\n";
$h{'Wall'} = 'only once';
$h{'Smith'} = 'more than once';
$h{'Smith'} = 'more than twice';
$h{'Smith'} = 'three times';
my $cnt = $x->get_dup("Wall") ;
print "Wall occurred $cnt times\n" ;
my %hash = $x->get_dup("Wall", 1) ;
print "Larry is there\n" if $hash{'Larry'} ;
print "There are $hash{'Brick'} Brick Walls\n" ;
my @list = sort $x->get_dup("Wall") ;
print "Wall => [@list]\n" ;
@list = $x->get_dup("Smith") ;
print "Smith => [@list]\n" ;
@list = $x->get_dup("Dog") ;
print "Dog => [@list]\n" ;
particularly interesting is this tidbit
$count = $x->get_dup($key) ;
@list = $x->get_dup($key) ;
%list = $x->get_dup($key, 1) ;
Look ma', I'm on CPAN.
** The Third rule of perl club is a statement of fact: pod is sexy.