Yeah, the Tk::after and Tk::repeat modules can be a source of confusion for a beginner. There is no perldoc for Tk::repeat, yet it is what is most often used. If you read perldoc Tk::after, you will see

 $id = Tk::After->new($widget, tid, $time, ’once’,   callback);
 $id = Tk::After->new($widget, tid, $time, ’repeat’, callback);
 $id->cancel;
 
once or repeat specifies whether the timer is a one-time-after
event, or a repeating repeat event.
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It just became a shortcut to write

$id = $mw->repeat( 10, sub{} );
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As far as your script goes, you might want to checkout the listbox -textvariable option to tie an array

#!/usr/bin/perl -w
use strict;
use Tk;

my $mw = Tk::MainWindow->new( -title => 'Listbox' );

my @array = ('A'..'Z');

my $lb = $mw->Scrolled('Listbox',
             -listvariable => \@array,
             )->pack();


my $b  = $mw->Button(
    -text    => 'Delete B',
    -command => sub{ del_it('B') },
)->pack();

MainLoop;

sub del_it {
    my $remove = shift;
    @array = grep { $_ ne 'B' } @array;
}

##################################################################
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that way, you can just load your array from the file

open FH, '< myfile' or warn "$!\n" ;                    
my (@array) = <FS>;                                               
close FH;
$listbox->update;  # may not be needed
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