sub rndStr{ join '', @_[ map{ rand @_ } 0 .. shift ] };

$! = 1;

( $_ % 1000 or printf( "\r$_ : %s   ", times ) ) 
    and $h{ rndStr 8, 'a'..'z', 'A' .. 'Z', 0 .. 9 } = undef
    for 0 .. 10_000_000;

5858000 : 80.703