Did you make a pasting mistake when posting your code, or are you really trying to have the "grid()" call for "$pg->Label()" include the creation of the BrowseEntry widget? That makes no sense. I would expect the for loop to look something like this:

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++;
}
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There may still be more things wrong with this, depending on what the @file_fieldnames array is supposed be... Why would you put the same set of field names into several BrowseEntry widgets?