That seems to work. Most helpful, thank you. Here is the updated PerlTk program (tested on NetBSD 1.6.1_STABLE)

#!/usr/pkg/bin/perl 
# gus_kbds.pl version 1.01
# Copyright (c) 2003 by Gan Uesli Starling

use Tk;
use strict;

use vars qw( 
  $programo_1 
  $xmodmap_verda $xmodmap_alia 
  $e_feedback $feedback 
  $pid_1
);

sub set_defaults {
 $xmodmap_verda = "/home/aplonis/.Xmodmap_verda";
 $xmodmap_alia = "/usr/X11R6/lib/X11/etc/xmodmap.std";
 $programo_1 = "xterm -u8 -fn -misc-fixed-medium-r-normal--14-130-75-75-c-70-iso10646-1 -name FOO";
}

set_defaults; # Do right away.

# Give initial feedback hint.
$feedback = "Premu butonon.";

# The main window.
my $mw_kbd = MainWindow->new(-title=>'Klavo-sxangxilo');

# Frame for buttons.
my $fm_kbds = $mw_kbd->Frame(-relief=>'flat',-borderwidth=>5);
my $bn_tajpu = $fm_kbds->Button( 
  -relief=>'raised',-text=>"Tajpu",-command=>\&tajpu,-width=>5,
  -background=>'gray',-activebackground=>'orange');
my $bn_eo = $fm_kbds->Button( 
  -relief=>'raised',-text=>"Verda",-command=>\&kbd_eo,-width=>5,
  -background=>'gray',-activebackground=>'green');
my $bn_en = $fm_kbds->Button(
  -relief=>'raised',-text=>"Alia",-command=>\&kbd_en,-width=>5,
  -background=>'gray',-activebackground=>'blue');
my $b_cls = $fm_kbds->Button(
  -relief=>'raised',-text=>"Finu",-command=>\&quit_MainLoop,
  -background=>"gray",-activebackground=>"red");
$bn_tajpu->pack(-side=>'left',-expand=>1,-fill=>'x');
$bn_en->pack(-side=>'left',-expand=>1,-fill=>'x');
$bn_eo->pack(-side=>'left',-expand=>1,-fill=>'x');
$b_cls->pack(-side=>'left');
$fm_kbds->pack(-side=>'top',-expand=>0,-fill=>'x');

################################
# BEGIN PROBLEM AREA -- SOLVED #
################################

sub tajpu {
    local $SIG{CHLD} = 'IGNORE';   # don't wait for child
                                   # exit status. avoids
                                   # zombies.
    die "fork failed: $!" unless defined($pid_1 = fork);
    return if $pid_1;
		$feedback = $programo_1;
    exec("$programo_1");
    warn "exec failed: $!";
    CORE::exit(1);
}

##############################
# END PROBLEM AREA -- SOLVED #
##############################

sub kbd_eo {
 $feedback = 'Klavaro verdas!';
 `xmodmap $xmodmap_verda`;
}

sub kbd_en {
 $feedback = 'Klavaro malverdas.';
 `xmodmap $xmodmap_alia`;
}

# Frame for info.
my $fm_btm = $mw_kbd->Frame(-relief=>'flat',-borderwidth=>5);
my $e_feedback = $fm_btm->Entry(
  -width=>20,	
  -textvariable=>\$feedback,-font =>'courier',
  -background=>"white",-foreground=>'blue');
$e_feedback->pack(-side=>'left',-expand=>1,-fill=>'x');
$fm_btm->pack(-side=>'bottom',-expand=>0,-fill=>'x');

# Backup results and quit program.
sub quit_MainLoop {
  $mw_kbd->destroy() if Tk::Exists($mw_kbd);
}

MainLoop;

And, just for completeness, here is the xternal Esperanto file ~/.Xmodmap_verda which Perl feeds to xmodmap for XFree86. It was created with the /usr/pkgsrc/x11/xkeycaps package.

!
! This is an `xmodmap' input file for 
!   PC 101 key, thin Delete, tall Enter (XFree86; US) keyboards.
! Automatically generated on Thu May 29 01:33:57 2003 by aplonis with
! XKeyCaps 2.46; Copyright (c) 1999 Jamie Zawinski <jwz@jwz.org>.
! http://www.jwz.org/xkeycaps/
!
keycode 0x6F =	Print	Sys_Req
keycode 0x18 =	ccircumflex	Ccircumflex
keycode 0x19 =	ubreve	Ubreve
keycode 0x1D =	jcircumflex	Jcircumflex
keycode 0x22 =	gcircumflex	Gcircumflex
keycode 0x23 =	hcircumflex	Hcircumflex
keycode 0x35 =	scircumflex	Scircumflex
keycode 0x40 =	Alt_L	Meta_L
keycode 0x71 =	Alt_R	Meta_R

I will write up a howto for this and post it first on the following URL:

starling.us/gus_netbsd

...then sometime later, translate it for...

esperanto.us