the way to do what you want is as follows:

my @keynames = qw/first second third fourth/;

my %hash = map { $_, 0 } @keynames;
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you can initialize a hash with values from an array as follows (this is called a hash slice):

my @keynames = qw/first second third fourth/;
my @squares = map {  $_ ** 2 } (1 .. 4);

my %hash;
@hash{@keynames} = @squares;
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also, in your example, i don't think the hash is totally empty. it should have a single undefined element in $hash{0}. i believe this is because the map variant you were using was treating the final value from your list (i.e. '0') as the map code and overwriting that element in the hash with undef for every element in @keynames.