#1/ Perl for loop:
my %x;

for my $i (0 .. 4) {
    $x{$i} = "x";
}

#2/ For as a statement modifier
$x{$_} = 'x' for 0 .. 4;

#3/ map generating hash key/value pairs:
%x = map {$_ => 'x'} 0 .. 4;

#4/ hash slice and x operator:
@x{0 .. 4} = ('x') x 5;