"{" 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 its dealing with based 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 "+" to give perl
               some help:

Since PBP "map EXPR, LIST" fell completely out of my scope, but it explains why a hash-ref could even be considered valid here.