Enhance the creation of the BrowseEntry slightly, like this:

my $browseval;
$mw->{a} = $mw->{main}->BrowseEntry(-label => "Select one:",
                                    -variable => \$browseval,
                                    )->pack;
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Now, you don't need a sub to return the value -- as soon as the user selects something in the widget, that value is assigned to the scalar whose reference was passed to the "-variable" option.