Then you have a broken Perl.

$ perl -e'use warnings; my %keys={A=>"b"};'
Reference found where even-sized list expected at -e line 1.

$ perl -v

This is perl, v5.10.0 built for i686-linux

Copyr...
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I still am fuzzy about the difference between “square brackets,” “braces,” and “parentheses.”

Square brackets create an array and return a reference to the array.
Curly brackets create a hash and return a reference to the hash.
Parens are like in Math. They change the order of operations.

From that, it follows that one never uses [] or {} when initialising an array or hash*, and that parenthesis aren't related to array or hash initialisation.

* — They might be used in initialising the value of an array or hash element's value, though.

Update: I added the last paragraph and the associated "*". I thought it was obvious, especially in the context of the other replies, but it might need saying.

In the context of this discussion, wouldn't it be that parenthesis indicate a list?

Elda Taluta; Sarks Sark; Ark Arks

In the current context, they definitely do NOT create a list. They override the relative precedence of the comma and assignment operators.

my %keys = (A => "b");
            -    ---
            1     2     List has two items
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my %keys = A => "b";
------------    ---
     1           2      List still there without parens
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Parens never create a list as far as I'm concerned, but people have debated me on two specific cases:

• In some places where a list is already being created, «()» is needed to indicate the list contains no elements. The parens don't create the list (since it would also get created if one used «@a» instead of «()»), they simply indicate the list is empty.

• In some places, the presence of parens affect the choice of operator to one that creates a list. (e.g. «$x = f();» vs «($x) = f();»). It's actually the assignment operator (not the parens) that creates the list as is gets created for «@a = f();» as well. Although both indirectly create a list, it makes no sense to say that parens create a list since noone says "«@a» creates a list".

Parens, curlies and square brackets have other uses than those I described, of course, but I doubt the other uses create confusion.

Lists value are created by operators that always take a list (e.g. foreach, list assignment, function calls, etc), and list literals are created by the comma operator.

foreach ($x) { }     @a = @b;     f $x;
         ^^          ^^   ^^        ^^
        list        list  list     list
        value      value  value    value

$i, $j;     @a = ($i, $j);
^^^^^^      ^^   ^^^^^^^^
 list      list  list value
literal   value  (from list literal)
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Anonymous in the sense that there is no named variable for the hash or array -- you have to access them via the reference.

What does "indicates a list" mean?

my %h = ($x);  # List created for RHS.
my %h = $x;    # List created for RHS.
my $s = ($x);  # No list created for RHS.
my $s = $x;    # No list created for RHS.
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