Lua: recursive function builts wrong table - pil4 - recursion

Question

while working on the exercise 2.2 of "programming in Lua 4" I do have to create a function to built all permutations of the numbers 1-8. I decided to use Heaps algorithm und made the following script. I´m testing with numbers 1-3.
In the function I store the permutations as tables {1,2,3} {2,1,3} and so on into local "a" and add them to global "perm". But something runs wrong and at the end of the recursions I get the same permutation on all slots. I can´t figure it out. Please help.
function generateperm (k,a)
if k == 1 then
perm[#perm + 1] = a -- adds recent permutation to table
io.write(table.unpack(a)) -- debug print. it shows last added one
io.write("\n") -- so I can see the algorithm works fine
else
for i=1,k do
generateperm(k-1,a)
if k % 2 == 0 then -- builts a permutation
a[i],a[k] = a[k],a[i]
else
a[1],a[k] = a[k],a[1]
end
end
end
end
--
perm = {}
generateperm(3,{1,2,3}) -- start
--
for k,v in ipairs (perm) do -- prints all stored permutations
for k,v in ipairs(perm[k]) do -- but it´s 6 times {1,2,3}
io.write(v)
end
io.write("\n")
end
debug print:
123
213
312
132
231
321
123
123
123
123
123
123