my @ch = (' ', '0'..'9', 'A'..'Z');
my $base = @ch;

my $num = 2_565_726_408;
my $s = '';

while ($num > 0) {
    $s = $ch[$num % $base] . $s;
    $num = int($num / $base);
}

print $s;  # ZZZZZZ