use strict;
use warnings;
use constant MAXBYTES => scalar 5;

my $result;

$| = 1;
my $msg = "Please be aware maximum length 255 characters!:\n"; 
    
do {
    print "Send this text to clients:\n\n";
    chomp ($result = <STDIN>);
    if (length($result) >= MAXBYTES) {
         print "\nYour input has a length of " . length($result) . "; the limit is ". MAXBYTES, "\n\n";
         print $msg;
    }
} until (uc ($result) eq "END");
$| = 1;