For small numbers, this should be straightforward ...

@divisors = grep { $number % $_ == 0 } 1 .. $number;
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I would not worry about optimizing this, unless you need it for very many or very big numbers ... but then it would be something like this:

# First, the lower half of the divisors:
@divisors = grep { $number % $_ == 0 } 1 .. sqrt($number);
# Then, the upper half:
push @divisors, map {$number == $_*$_ ? () : $number/$_} reverse @divi
+sors;
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(Sorry, I missed the Math::BigInt stuff ... but I guess the algorithm above can be adapted to that case as well?)

Update: Most of my code could benefit from comments. I added comments to the optimized version. Never code as if the guy who ends up maintaining your code can always maintain a proper caffeine intake.