From map:

{ starts both hash references and blocks, so map { ... could be either the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look ahead for the closing } it has to take a guess at which it's dealing with based on what it finds just after the {. Usually it gets it right, but if it doesn't it won't realize something is wrong until it gets to the } and encounters the missing (or unexpected) comma. The syntax error will be reported close to the }, but you'll need to change something near the { such as using a unary + or semicolon to give Perl some help:

$ perl -wMstrict -le 'print map { (2, $_) } 1 .. 2'
2122
$ perl -wMstrict -le 'print map {; 2, $_ } 1 .. 2'
2122
$ perl -wMstrict -le 'print map { +2, $_ } 1 .. 2'
2122
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>   perl -wMstrict -le 'print map { +2, $_ } 1 .. 2'

I'm not a big fan of the unary plus solution, since it's not easily understandable why the unary plus sometimes works or not.

Fun with Perl's parsing:

 DB<100> map {+2,$_} 1..3   # +2 is not a hash key
 => (2, 1, 2, 2, 2, 3)

 DB<101> map {2,$_} 1..3    # 2 is a hash key 
           ;;
Not enough arguments for map at (eval 21)[multi_perl5db.pl:644] line
2, near "} 1"
syntax error at (eval 21)[multi_perl5db.pl:644] line 2, near "} 1"

  DB<102> map {2,$_}, 1..3  # 2 is a hash key
 => ({ 2 => 1 }, { 2 => 2 }, { 2 => 3 })

  DB<103> map {+2,$_}, 1..3 # +2 is not a hash key
           ;;
syntax error at (eval 26)[multi_perl5db.pl:644] line 2, near "},"

  DB<104> map {+2 => $_}, 1..3 # +2 is not a hash key even with fat co
+mma
           ;;
syntax error at (eval 29)[multi_perl5db.pl:644] line 2, near "},"
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so far so consistent, BUT

  DB<105> map +{+2 => $_}, 1..3 # +2 IS A HASH KEY
 => ({ 2 => 1 }, { 2 => 2 }, { 2 => 3 })

  DB<106> map +{+2 , $_}, 1..3  # +2 IS A HASH KEY even without =>
 => ({ 2 => 1 }, { 2 => 2 }, { 2 => 3 })
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see my point? :)

Cheers Rolf
_{(addicted to the Perl Programming Language and ☆☆☆☆ :)
Je suis Charlie!}

I'm not a big fan of the unary plus solution

I agree, I was just keeping the examples in line with what the map docs were saying. Personally in this case I'd prefer map { (...) } ..., since it makes it look obvious that returning a list is intentional. OTOH I also use map {$_=>1} ... very often, but that one doesn't cause any parse trouble.

The map +{...}, ... stuff is also mentioned in the map docs, although I haven't yet seen it used in the wild - personally I'd probably write map { {...} } .... As you can tell I usually prefer map BLOCK LIST over map EXPR, LIST.

... to force an anon hash constructor use +{:

my @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs
                                          # comma at end
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to get a list of anonymous hashes each with only one entry apiece.

I recently had a similar problem which I solved by changing to the EXPR,LIST form of map. Thanks for the explanation.

C:\Users\Bill>perl -wMstrict -le "print map( do{2,$_}, 1..2)"
2122
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Bill

Far simpler way to disambiguate:

{; ... }        # BLOCK
+{ ... }        # Hash constructor
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Nice idea to use 'do{ }' sentence, I like to use it. So I'll use 'do{ }' in such situations.

Thanks ;)