What do you want to do here?

The idea is to add the number of days per month to $jul - but what you are trying is neither a loop nor a sum, but some kind of array slice (that would be written as @Days[0 .. $mo]).

Maybe you meant something like this:

use List::Util qw(sum);
...
$jul = sum(@Days[0..$mo]) + $da;
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By the way please try to write in proper english words, we all try (and we're not all native speakers)