Your question has already been answered so I thought I'd throw in another way (there are more) to accomplish summing two cubes...

sub sum2cubes {
  my @cube = @_; # more meaningful than $a and $b
  return($cube[0] ** 3 + $cube[1] ** 3);
}
print sum2cubes(3,4);
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Simplicity is one of Perl's finer points.