package DO_STUFF.pm;

BEGIN {
  use strict;
  use Exporter;
  use vars qw(@ISA @EXPORT);
  my $conf = determine_conf_module_name();
  eval "use $conf;";
}
do_stuff....etc...

##</code><code>##

  my $conf = determine_conf_module_name();
  eval "use $conf;";

##</code><code>##

package DO_STUFF.pm;

use strict;
use Exporter;
use vars qw(@ISA @EXPORT);
...
sub new {
  my $conf = determine_conf_module_name();
  eval "use $conf;";
...
}