my $string = "foo:bar:baz:quux";

for (1..10) {
    print "$_\n";
    if ($string=~m/:/g || length($string)!=16)
    {
        print "Either : already in string on the length is invalid\n";
    }
}

__END__
1
Either : already in string on the length is invalid
2
Either : already in string on the length is invalid
3
Either : already in string on the length is invalid
4
5
Either : already in string on the length is invalid
6
Either : already in string on the length is invalid
7
Either : already in string on the length is invalid
8
9
Either : already in string on the length is invalid
10
Either : already in string on the length is invalid