my ($row,$mfld) = (0,0);
for my $field ( @file_fieldnames ) { # (and where does this array come from?)
    $pg->Label(....)->grid(-row => $row, -column => 0);
    my $brb = $pg->BrowseEntry(-variable => \$map_fields[$mfld++],
                               )->grid(-row => $row, -column => 1);
    $brb->insert('end', @$dbfields);
    $row++;
}