You may try something like this (tested) example

#!/usr/bin/perl
use strict;
use warnings;
use IO::File;
use Mail::Internet;


defined $ARGV[0]
  or die 'Usage: perl emailprogramma3.txt perltst\n';
my $file = $ARGV[0];
my $fh   = new IO::File;

# See File::Spec catpath method
my $path = '/where/is/perltst/' . $file;

$fh->open("< $file") or die "Can't open $file\n";

my $mail = Mail::Internet->new($fh);
$mail->print_body;

$fh->close;

__END__

Output :
------

Content-Type: text/plain;
name="Niet zelf een nummer bedenken!.txt"
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
filename="Niet zelf een nummer bedenken!.txt"

SW4gZfafajkGAGEQRda789UIOKAJkljfqj8q8gdG9ldm9lZ2VuGAOGJHAFfagkl4538daj
+a
a3QgaGllcnZvb3IgQWRkSXRlbSBvZiBkZgBzdGFuZGFhcmQgcHJvZ3JhbW1hJ3MgdmFuIE
+lC
Qiag6FGIJ891rgahuClJCQQ==
--------------000002060003020904090008--
[download]

hth,
PooLpi