I use dbmopen to create/open this dbm database file.
The version of perl is 5.8.6 Activeperl.

You're probably using DB_File. Run

perl -MAnyDBM_File -le"print for @AnyDBM_File::ISA"
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to verify.

DB_File (or specifically DB_HASH, which is what you're using) doesn't allow multi-valued keys, so chances are you've misdiagnosed the situation. Insert the following snippet and examine your database

use Data::Dumper;
$Data::Dumper::Indent=1;
$Data::Dumper::Useqq=1;
die Dumper( \%DATABASE );
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you'll be suprised what you find.

Unless you have a good reason to use dbmopen, you should switch to the tie interface.