There seems to be some weird interaction when using -textvariable. I don't know what is exactly causing it, but here are 2 solutions. The first is just avoid using -textvariable. Uncomment the -textvariable line and see how it breaks the following. The second example uses padding.

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

my $plaintext;
my $mw = MainWindow->new();

my $password_e = $mw->Entry( 
#    -textvariable => \$plaintext,
    -show => '*',
    -width   => '10'
)->pack( -side => 'top');

my $password_b = $mw->Button(
        -text    => "Encrypt",
        -width   => '13',
        -command => \&encrypt
    )->pack( -side => 'top', -pady => 10 );

sub encrypt {
    $plaintext = $password_e->get();
    
    my $key = pack("H16", "0123456789ABCDEF");
    print "text->$plaintext\n";

    my $cipher = Crypt::CBC->new(
    -cipher => 'DES',
    -key    => $key,
    );

    my $ciphertext = $cipher->encrypt($plaintext);
    print("$ciphertext\n");

    my $recovered = $cipher->decrypt($ciphertext);
    print("$recovered\n");

}

MainLoop
[download]

if you absolutely need to use -textvariable, you need to pad

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

my $plaintext;
my $mw = MainWindow->new();

my $password_e = $mw->Entry( 
    -textvariable => \$plaintext,
    -show => '*',
    -width   => '10'
)->pack( -side => 'top');

my $password_b = $mw->Button(
        -text    => "Encrypt",
        -width   => '13',
        -command => \&encrypt
    )->pack( -side => 'top', -pady => 10 );

sub encrypt {

    my $key = pack("H16", "0123456789ABCDEF");
    print "text->$plaintext\n";
    
   #this will allow you to use -textvariable
    $plaintext = "\0" x ( 8 - length($plaintext)%8 ) . $plaintext;

    my $cipher = Crypt::CBC->new(
    -cipher => 'DES',
    -key    => $key,
    );

    my $ciphertext = $cipher->encrypt($plaintext);
    print("$ciphertext\n");

    my $recovered = $cipher->decrypt($ciphertext);
    print("$recovered\n");

}

MainLoop
[download]