If you don't want to prepend a space to each element in the list, use a more advanced type of a list. For example, a Tk::HList:

#! /usr/bin/perl
use warnings;
use strict;

use Tk;
use Tk::HList;

my $mw = 'MainWindow'->new(-title => 'Listbox test');
my $hl = $mw->HList(-drawbranch => 0,
                    -pady       => 0,
                    -padx       => 10  # <- This is what you need!
)->pack;

for ('A' .. 'H') {
    $hl->addchild("", -text => $_);

}

MainLoop();
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