my @keys = qw{a b c d};
my %hash = map { $_ => 1 } @keys;

##</code><code>##

my @keys = qw{a b c d};
my %hash;
$hash{$_} ++ for @keys;

##</code><code>##

my @keys = qw{a b c d};
my %hash;
@hash{@keys} = (1) x scalar @keys;