Try the leave_curses() method.

#!/usr/bin/perl

use strict;
use warnings;
use Curses::UI;

my $cui = new Curses::UI( -color_support => 1, );

my $window = $cui->add(
    "main_window",
    "Window",
    -width  => 50,
    -height => 4,
);

my $text_editor = $window->add(
    "description_text",
    "TextEditor",
    -text         => "",
    -title        => "Enter Description",
    -border       => 1,
    -singleline   => 1,
    -width        => 40,
    -bfg          => "red",
    -titlereverse => 0,
    -showlines    => 1,
    -x            => 0,
    -y            => 0,
);

my $okay_button = $window->add(
    "buttons",
    "Buttonbox",
    -border  => 1,
    -width   => 6,
    -bfg     => "blue",
    -x       => 40,
    -y       => 0,
    -buttons => [
        {
            -label    => "Okay",
            -value    => 1,
            -shortcut => "o",
            -onpress  => sub { $cui->mainloopExit; },
        },
    ],
);
$text_editor->focus;
$cui->leave_curses;
print "Outside of mainloop!\n";
$cui->mainloop;
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