Not sure how you used Tk::Entry. If you use the textvariable option of Tk::Entry, it becomes much more simple for you to set/get the content of Tk::Entry (avoid passing params to your callback function, and do all those handling by yourself, that is more complex):

use Tk;
use strict;

my $entry_val = "initial value";
my $mw = new MainWindow(title => "demo");
$mw->Entry(width => 20, textvariable => \$entry_val)->pack;
$mw->Button(text => "click", command => sub {print $entry_val, "\n";})
+->pack;
MainLoop;
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