Other than the already suggested

@tapes_for_box1 = @unique_vault_list[0..19];
@tapes_for_box2 = @unique_vault_list[20..$#unique_vault_list];
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you could use

@tapes_for_box1 = splice(@unique_vault_list, 0, 20);
@tapes_for_box2 = @unique_vault_list;
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which can be expanded into

my @tapes_by_box;
push(@tapes_by_box, [ splice(@unique_vault_list, 0, 20) ])
   while @unique_vault_list;
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The difference is that splice removes the items from @unique_vault_list. (And it does so efficiently when removing from the start or end, as is the case here.)