strat almost had it...... the -anchor=>'w' option belongs in the label options, not the packing options.

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

my $mw = MainWindow->new;

my $lbl = $mw->Label(  # File 
             -text => '(start)',
             -width => 12,
             -background => 'bisque',
             -justify => 'left',
              -anchor =>'w',
             -relief => 'groove',
          )->pack;

my $btn = $mw->Button(
             -text => "Go!",
             -command => \&test,
          )->pack;

MainLoop;

sub test {
   $lbl->configure(-text => 'A');
}
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