I don't have a theoretical answer about the source of the problem; GDBM seems relatively undocumented. But I do have a workaround.

Define your file
$voo = "/somewhere/voo";
somewhere before the subroutine call.

Substitute the variable $voo for $_[0] in the DBMopenread subroutine -- it is a global variable, so the statement become:
$DBREFread=tie %DBM, GDBM_File, $voo, &GDBM_READER, 0644;

Leave the parameter out of the subroutine call, just:
&DBMopenread;

Seems to work. Of course you have to pay attention to the other instances of 'voo' versus the variable $voo.

Sorry I don't have a more satisfactory "why," but you can get on with yur work.