Works for me. It keeps asking until I enter "end" or "END" (or "eNd" etc.). 
#!/usr/bin/perl
use warnings;
use strict;

use constant MAXBYTES => 5;

$| = 1;

MSG: print "Please be aware maximum length ${\MAXBYTES} characters!:\n
+";

my $result;
do {
    print "Send this text to clients:\n\n";
    chomp ($result = <STDIN>);
    if (length($result) >= MAXBYTES) {
        print "\nYou have reached the limit of characters ",
              length $result,
              "!\n\n";
        goto MSG
    }
} until uc $result eq "END";

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لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

In reply to Re^3: do until loop breaks before meeting the condition by choroba
in thread do until loop breaks before meeting the condition by thanos1983

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