Seconded on the using floats as hash keys being a bad idea. Run this version to see why:

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";
}
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Perhaps if you step back and described what you're trying to accomplish with this hash . . .