Create an array of indices sorted by @b and then use it to reorder both arrays:

@a = (1,5,3);;
@b = (5,2,1);;
@sortedIndices = sort{ $b[ $a ] <=> $b[ $b ] } 0 .. $#b;;

@a = @a[ @sortedIndices ];;
@b = @b[ @sortedIndices ];;

print "@a\n@b";;
3 5 1
1 2 5
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