DB<15> $, =","; say unpack "A1A11A*",unpack "B*", pack "F>", 1/5
0,01111111100,1001100110011001100110011001100110011001100110011010

  DB<16> $, =","; say unpack "A1A11A*",unpack "B*", pack "F>", 1/15
0,01111111011,0001000100010001000100010001000100010001000100010001

##</code><code>##

sub period_length { 
    my $q = shift ; 

    until ( $q % 2 ) { $q /= 2 }; 
    my $r=1; 
    # say "q:$q"; 
    for (1..$q) { 
        $r = $r * 2 % $q; 
        return $_ if $r == 1; 
        # say "r:$r" 
    } 
}