$c is incremented only after the second value is stored into the array,

That's not true. The order in which things happen:

1. $c
2. $c
3. post-increment
4. my @a
5. list assign

@a doesn't even exist at the point you say values are assigned to it.

>perl -MO=Concise,-exec -e"my @a = ( $c, $c++ );"
1  <0> enter
2  <;> nextstate(main 1 -e:1) v:{
3  <0> pushmark s
4  <#> gvsv[*c] s                     # $c
5  <#> gvsv[*c] s                     # $c
6  <1> postinc[t4] sK/1               # post-increment
7  <0> pushmark s
8  <0> padav[@a:1,2] lRM*/LVINTRO     # my @a
9  <2> aassign[t5] vKS/COMMON         # list assignment
a  <@> leave[1 ref] vKP/REFC
-e syntax OK
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