You seem to be setting the -anchor of a widget to "sec" which is not a valid value.

Instead of editing Tk/After.pm, you can redeclare the method from your code:

use Data::Dumper;

my $orig = *Tk::After::once{CODE};
local *Tk::After::once = sub {
    warn Dumper \@_;
    $orig->(@_)
};
[download]

You should see something like

...
$VAR1 = [
          bless( [
                   bless( {
                            '_After_' => {
                                           'after#2' => $VAR1->[0]
                                         },
                            '_TkValue_' => '.frame.message',
                            '_Destroy_' => [
                                             bless( [
                                                      sub { "DUMMY" },
                                                      $VAR1->[0][0]
                                                    ], 'Tk::Callback' 
+)
                                           ]
                          }, 'Tk::Message' ),
                   'after#2',
                   1000,
                   'once',
                   bless( [
                            sub { "DUMMY" }
                          ], 'Tk::Callback' )
                 ], 'Tk::After' )
        ];
[download]

In this case, the widget was a Tk::Message. I fear yours will be a Frame, as that's the type of $w, but maybe the additional information will help you identify it further.