Oh! Silly me. You call:

main('90   35  29 + 90  24 29')
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But main is:

sub main {
    reduce(@ARGV);
}
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so reduce doesn't see the parameters passed to main, it sees @ARGV which is empty. Change main to:

sub main {
    reduce(@_);
}
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Also:

1. Whatever is natural for the sub. Because of its recursive implementation returning anything except a list from reduce doesn't make sense.
2. There are two different test modes: Functional testing where you test the whole system works as a whole - call main, and unit testing where you test individual units work as advertised - call reduce.

In this case "the whole" is a trivial wrapper around reduce so I'd go for testing reduce directly - a unit test.