Difference between returning +{} or {} in perl from a function, and return ref or value

Question

I just started work with a new team new place. They told me we return ref instead of value in our perl modules. Also I saw something like return +{foo=>'bar'}; and return {foo=>'bar'}; Whats the difference? And whether to return ref or value?

Answer 1

The + in return +{foo=>'bar'} is completely useless.

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First, some background.

The Perl language has ambiguities. Take for example

sub f {
   {  }     # Is this a hash constructor or a block?
}

{ } is valid syntax for a block ("bare loop").
{ } is valid syntax for a hash constructor.
And both are allowed as a statement!

So Perl has to guess. Perl usually guesses correctly, but not always. You can give it "hints". Unary-+ can be used to do this. Unary-+ is a completely transparent operator; it does nothing at all. However, it must be followed by an expression (not a statement). { } has only one possible meaning as an expression.

+{ }   # Must be a hash constructor.

Similarly, you can trick Perl to guess the other way.

{; }   # Perl looks ahead, and sees that this must be a block.

Here's an example where Perl guesses wrong:

map { {} } 1..5   # ok. Creates 5 hashes and returns references to them.
map {}, 1..5      # XXX Perl guesses you were using "map BLOCK LIST".
map +{}, 1..5     # ok. Perl parses this as "map EXPR, LIST".

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As for the code in the question, return must be followed by an expression (if anything), so there's only one possible interpretation for return { ... };, so the + is completely useless there.

Most people only disambiguate when necessary. Others might add + whenever it's ambiguous (even if Perl would guess right). But this is the first time I've heard of using + in front of every hash constructor.