Your using a 'fat comma' (aka the "quoting comma": =>)

That is a very confusing statement.

From the mouth of the Camel:

The => operator is just a synonym for a comma, but it's more visually distinctive and also quotes any bare identifiers to the left of it.

$_ not being a bare identifier, the 'fat comma' is just a comma here.

Actually my %h = map { "$_" , "$_" } @arr ; gives the same syntax error as my %h = map { "$_" => "$_" } @arr ; proving that the 'fat comma' has nothing to do with the error. It are the double quotes that confuse the interpreter.