The math means that it constructs the hash from:

• all the elements of @_ if there are an even number;
• all of them except the last one if there are a odd number;

print "$_ :", $_ + $_ % 2 -1 for 0 .. 11;;
0 : -1
1 : 1
2 : 1
3 : 3
4 : 3
5 : 5
6 : 5
7 : 7
8 : 7
9 : 9
10 : 9
11 : 11
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(Remember the zeroth element!).

The purpose (I guess) is to avoid the "Odd number of elements in hash assignment" warning,