Hash slices are useful, but assigning a scalar to them is less useful. Perl could warn in that case, but I don't know that it should.

@hash{@array} = 'foo'; # converted to a 1-element list
@hash{@array} = ('foo'); # equivalent
@hash{@array} = ('foo', undef, undef); # equivalent
@hash{@array} = ('foo', 'foo', 'foo'); # arguably should be equivalent
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