sub use_str
{
    my $re = qr/^1\d2\d3\d4\d5\d6\d7\d8\d900$/;

    for (my $n = 1_010_101_030; $n <= 1_389_026_623; )
    {
        # stringify the qr regex
        my $str = "$re";
        my $s  = $n * $n;
        return   $n if $s =~ /$str/; # recompiled each time I guess?
           $n += 40;
           $s  = $n * $n;
        return   $n if $s =~ /$str/;
           $n += 60;
    }

    die;
}