Your variable name, $days_mask, gives a clue. You are working with a bitmask. As Zaxo pointed out, you need to use a bitwise-and to extract the value of individual bits. If you just want the day numbers you might do it like this:

 
my @day_numbers = grep $days_mask[$i] & 2**$_, 0..6;
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-sauoq
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