Curly braces can denote a block or an anonymous hash, and map can take a block or an expression, so map followed by a curly brace is ambiguous. Unfortunately, perl doesn't look forward to the closing brace to see whether it's a block or an anonymous hash, so perl has to guess.

If the opening curly brace is followed by a string and a comma (, or =>), perl guesses it's an anonymous hash. Anything else, and perl guesses it's a block. This heuristic works most of the time, but in some cases perl guesses wrong: 

# a block returning a list of two elements,
# but perl thinks it's an anonymous hash
%hash = map { "$_", 2 } @l;

# an anonymous hash, but perl thinks it's a block
@hash_refs = map { $_, 2 }, @l;

[download]
However, there's not much urgency to fix this problem, because it's so easy to avoid: 
# force parsing as a block
%hash = map { +"$_", 2 } @l;
%hash = map { ; "$_", 2 } @l;
%hash = map { ( "$_", 2) } @l;

# force parsing as an anonymous hash
@hash_refs = map +{ $_, 2 }, @l;

[download]

In reply to Re: map syntax bug-a-boo? by chipmunk
in thread map syntax bug-a-boo? by MeowChow

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