> For the complete documentation index, see [llms.txt](https://trizen.gitbook.io/perl6-rosettacode/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://trizen.gitbook.io/perl6-rosettacode/programming_tasks/m/modular_exponentiation.md).

# Modular exponentiation

This is specced as a built-in, but here's an explicit version:

```perl
sub expmod(Int $a is copy, Int $b is copy, $n) {
    my $c = 1;
    repeat while $b div= 2 {
        ($c *= $a) %= $n if $b % 2;
        ($a *= $a) %= $n;
    }
    $c;
}

say expmod
    2988348162058574136915891421498819466320163312926952423791023078876139,
    2351399303373464486466122544523690094744975233415544072992656881240319,
    10**40;
```

#### Output:

```
1527229998585248450016808958343740453059
```