See the -show option in the Tk::Entry man page. Example:

#!/usr/bin/perl

use warnings;
use strict;
use Tk;

my $input = '';

my $m = tkinit;
$m->Entry(
    -show           => "*",
    -textvariable   => \$input,
)->pack;
$m->Button(
    -text       => 'Quit',
    -command    => sub { $m->destroy },
)->pack;

MainLoop;

print "input was: $input\n";
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