Why does a syntax error occur when trying to initialize a hash using a variable?

Question

Perl

When trying to initialize the hash like this

$h{a} = "1";
$h{b} = "2";

An error occurs:

syntax error at p.pl line 93, near "$h{"
syntax error at p.pl line 94, near "$h{"

And if so:

$h->{a} = "1";
$h->{b} = "2";

then Perl doesn't swear.
Hence the question, when to initialize a hash using a variable, and when through a pointer to a hash?
Explain, please!

Answer 1

$a="index1";

$h{$a}="1";
$h{"index2"}="2";