in reply to Syntax for casting map to a hash or an array

Is there a reference dereference happening?

Yes - the curlies directly around the map create a hash reference from the list that map returns (see number 3 in Making References), and then the outer %{ } dereferences that hash reference (see number 2 in Using References).

So the inner pair is a code block.

Nope, it's a hashref constructor, but since it can also look like a block, Perl sometimes has to "guess" which one it is, and AFAIK that's where the warning is coming from.

If I need to return an array, as opposed to a list or a hash, what would that syntax be?

The same kind of reference-dereference operation, just in this case create an arrayref with [...] and dereference it with @{...}:

print Dumper @{ [ map {"<$_>"} qw/a b c/ ] }; __END__ $VAR1 = '<a>'; $VAR2 = '<b>'; $VAR3 = '<c>';

Although I think this is needed less often than the %{{...}} trick when it comes to passing stuff to functions, since most functions impose list context on their arguments anyway. However, it is sometimes used as a trick to interpolate things into strings that don't normally interpolate, so for example:

sub foo { return "World!" } print "Hello, ", foo(), "\n"; # prints "Hello, World!" print "Hello, foo()!\n"; # prints "Hello, foo()!" print "Hello, @{[ foo() ]}\n"; # prints "Hello, World!"

While in this example the first print looks cleaner than the last, the trick can sometimes be useful when interpolating things into long heredocs, for example.

Replies are listed 'Best First'.
Re^2: Syntax for casting map to a hash or an array
by cbeckley (Curate) on Apr 05, 2017 at 19:42 UTC

    Thank you again. A lot of information as always.

    It makes sense that there wouldn't be too many @{[...]]} running around in the wild.

    Thanks in particular for this:

    print "Hello, @{[ foo() ]}\n"; # prints "Hello, World!"