use strict;
use warnings;
use 5.012;

use Tk;

my $entry_text;


my $mw = MainWindow->new;

my $entry = $mw->Entry(
    -text => 'Enter answer here',
)->pack;

$entry->focus;
$entry->selectionRange(0, 'end');

sub my_validator {
    my $input = $entry->get();

    if ($input eq 'hello') {
        $entry->delete(0, 'end');
        $entry->insert(0, 'Try again!');
        $entry->selectionRange(0, 'end');
        $entry->icursor(0);
    }
    else {
        say "Thanks for the great input!";
        $entry_text = $input;
        say $entry_text;
    }
}

$mw->Button(
    -text       => 'Submit',
    -command    => \&my_validator,
)->pack;

MainLoop;