From perldoc perlfunc documentation of "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 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

This will parse correctly, forcing BLOCK:

my %hash = map {+ qq(stuff_$_) => $_ } qw(asdf qwert zxcv)
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