use warnings;
use strict;

##</code><code>##

foreach $i (1...20)   # that ... should be ..

##</code><code>##

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

my @a=qw/one two three four five six seven eight nine ten/;

my $mw = MainWindow->new;

$mw->geometry('200x200');

my $f = $mw->Scrolled("Frame",-borderwidth => 3, -relief => 'groove',-height=>20)
        ->pack(-expand=> 1, -fill=>'both');


foreach my $i (0 .. 19){  # this should be foreach my $i (@a), but it's your code :-)
print "$i\n";

my $c = $f->Checkbutton(-text=>$a[$i],-command=>[ \&fun, $a[$i] ] )->pack();
   $i++;
  }

MainLoop;

sub fun{
  print "@_\n";
}