You just can't do it - it doesn't make sense in Perl. The nearest thing to do what you intend (without using any modules) needs a loop:

   my $jul = $da;
   $jul += $_ for @Days[0..$mo-1];
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@ARRAY[LIST] is known as an array slice. It returns a list of elements of @ARRAY. You can also slice a hash. See Slices for the details.