I stripped both pieces of code down to the bare minimum of calculating just $max_workers (1st solution) and $auto_max_workers (2nd solution). I ran some benchmarks on these last night.

The "$max_workers (1st solution)" ran about 15 times faster than "$auto_max_workers (2nd solution)". This is not overly surprising: calculating $max_workers is mostly just arithmetic; calculating $auto_max_workers involves instantiating a temporary object and then some arithmetic.

In the absence of other advice, "$max_workers (1st solution)" will end up in Kens::MCE::Util (or whatever I decide to call it). I've implemented nothing as yet; however, expected usage would probably be something like:

...
use Kens::MCE::Util 'get_max_workers';
...
my $percent; # value from user input, config file, etc.
...
my $mce = MCE::->new(
    max_workers => get_max_workers($percent),
    ...
);
...
[download]