Just a guess... try this: (untested)

$tied_hash{list} = [1,2,3,4,5];
my $sync_dbm_obj = tie %tied_hash, 'MLDBM::Sync', '/tmp/mldbmsync', O_
+CREAT|O_RDWR, 0640;

# modify list
my $listref = $tied_hash{list};
push(@$listref,6);

# store list back into hash.
$tied_hash{list} = $listref;

# did it work?
@values = @{$tied_hash{list}};
print @values;
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-Blake