how would I make it so that it returns something when the result is zero

It currently puts 0 in @digits. Adjust the last line as desired.

@digits = 0 if !@digits;
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I just would rather not have to use a cpan module for such a simple task

Well, it was complex enough that you wanted help with it. I'm also convinced mine can give the wrong result due to floating point error.

my @digits;
do {
   unshift @digits, $num % 20;
   $num = ( $num - ( $num % 20 ) ) / 20;
} while $num > 0;
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