IMHO "solely" is wrong, for instance lexicals are initialized to undef and if you check the reference of a variable declared in the head of a loop you can observe that they can change, i.e. there is no singular allocation overhead at compile time.

Furthermore there is an opcode executed at runtime for this (see line3)

perl -MO=Concise -e 'my $a;$a=1'
8  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v:{ ->3
3     <0> padsv[$a:1,2] vM/LVINTRO ->4
4     <;> nextstate(main 2 -e:1) v:{ ->5
7     <2> sassign vKS/2 ->8
5        <$> const[IV 1] s ->6
6        <0> padsv[$a:1,2] sRM* ->7
[download]

Anyway this is one of the last places I would consider for optimizations.