Yes, I am using TK. I read Validation in Entry, but that gives you option of "focus" on entry box and "key". But what I need is a separate button for a user to validate it.

(For example, a user may play around with the text in Entry box, but he may not choose to conform to changes and might close the window. So, I want a submit button alongwith it. Only when it is pressed, I'll take the input.)

Such a functionality does not exist. You have to create a normal Tk::Button and implement the logic yourself in its -command.
لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

a user may play around with the text in Entry box, but he may not choose to conform to changes and might close the window. So, I want a submit button alongwith it. Only when it is pressed, I'll take the input.

You can also intercept the 'window close' event and change a textvariable to anything you want:

use strict;
use warnings;
use 5.012;

use Tk;

my $entry_text = 'Enter answer here'; #textvariable

#======Main window==========

my $mw = MainWindow->new;
$mw->protocol("WM_DELETE_WINDOW", \&my_quit);

sub my_quit {
    say "Window closing...$entry_text";
    $entry_text = '';
    say "-->$entry_text<---";
    exit;
}



#======Entry================

my $entry = $mw->Entry(
    -textvariable => \$entry_text,
)->pack;

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

sub my_validator {
    if ($entry_text eq 'hello') {
        $entry_text = 'Try again!';
        $entry->selectionRange(0, 'end');
        $entry->icursor(0);

    }
    else {
        say "Thanks for the great input!";
        say $entry_text;
    }
}

#========Button=================

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


MainLoop;
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