my @keys = qw( 0 0.1 0.3 1 3 10 30 1000 );
my %n;

for ( my $mag = 5.0 ; $mag < 9.0 ; $mag += 0.1 ) {
  @{ $n{ $mag } }{ @keys } = ( 0 ) x @keys;
}

for ( sort { $a <=> $b } keys %n ) { 
  print "mag: $_\t$n{$_}->{30}\n";
}