P[[bb]]
  $1: P
  $2: [bb]
x[xx[x]]
  $1: x
  $2 xx[x]
Z[[z]y]
  $1: Z
  $2: [z]y
X[a[s]x]
  $1: X
  $2: a[s]x
V[a[b[c[d]f]g]h]
  $1: V
  $2: a[b[c[d]f]g]h
[download]

Also to not forget V[a]C[b] (multiple tags in line) should be allowed as well. So the current one is quite nice (which does pretty much everything). But removing outermost pair of [] is pretty trivial (if done outside the already used regex).

my ($key,$value)=($1,$2);
$value=~s/^\[|\]$//g;
[download]

So as i said i can live with it :).