What's the difference between these two definitions?

Question

Why it's a syntax error:

my @hash{1..4}=(1..4);

but not this one:

my %hash;
@hash{1..4}=(1..4);

Answer 1

the 1st example is of a lexically scoped 'my' + a hash slice which pre-supposes that one can declare a hash in the manner of a slice which is not valid syntax. your 2nd example is appropriate, declaring the hash first, assuming that you're use'ing strict + warnings;

Answer 2

my requires a variable or a list of variable in parens as argument.

@hash{1..4}

is neither of those, so

my @hash{1..4}

is a syntax error.

Answer 3

First example fails, because hash slice is an operation that returns some result. Obviously, perpending it with my declaration makes no sense, just like writing something like my 2+2 wouldn't. my must be followed by list of variables to declare.

Second example does just that - declares a hash in current scope and then accesses a slice of it.