Just like about any other operator in Perl, map imposes a certain context on its operands, regardless of the context it itself is in. In:

map {$hash->{foo}} 1;
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there are actually two lists, both containing a single element: the first list consists of $hash->{foo}, the second of 1.

Think of it this way, if map would not provide list context to its inner block, the following wouldn't do what it does now:

%h = map {$_ => 1} qw[foo bar baz];
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