Thanks. I think I'm beginning to get it. It was easier for me to understand what's going on after I used Data::Dumper to dump the hash, which is actually a HoH (hash of hashes).

use warnings;
use strict;
use Data::Dumper;

my @keys = qw( 0 0.1);
my %n;

for ( my $mag = 5.0 ; $mag < 6.0 ; $mag += 0.1 ) {
  @{ $n{ $mag } }{ @keys } = ( 0 ) x @keys;
}

print Dumper(\%n);
__END__
Outputs:

$VAR1 = {
          '6' => {
                   '0.1' => 0,
                   '0' => 0
                 },
          '5.3' => {
                     '0.1' => 0,
                     '0' => 0
                   },
          '5.7' => {
                     '0.1' => 0,
                     '0' => 0
                   },
          '5.1' => {
                     '0.1' => 0,
                     '0' => 0
                   },
          '5.6' => {
                     '0.1' => 0,
                     '0' => 0
                   },
          '5.8' => {
                     '0.1' => 0,
                     '0' => 0
                   },
          '5.9' => {
                     '0.1' => 0,
                     '0' => 0
                   },
          '5.5' => {
                     '0.1' => 0,
                     '0' => 0
                   },
          '5.2' => {
                     '0.1' => 0,
                     '0' => 0
                   },
          '5.4' => {
                     '0.1' => 0,
                     '0' => 0
                   },
          '5' => {
                   '0.1' => 0,
                   '0' => 0
                 }
        };
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