This might be an easier way to avoid the floating-point problem for you hash keys:

# first, generate fixed-precision strings for magnitudes
# and bin values:

my @mag_keys = map { sprintf("%.1f", $_/10) } ( 50 .. 89 );
my @bins = qw/0 0.1 0.3 1 3 10 30 1000/;

# now initialize hash bins for counting:
# (this is probably unnecessary, unless you're re-using
# the hash on multiple separate data sets)

my %n;
for my $mag ( @mag_keys ) {
    for my $bin ( @bins ) {
        $n{$mag}{$bin} = 0;
    }
}
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The next thing to watch out for, when actually counting things up, is to avoid using the "==" operator to test whether a floating point value from your input data matches a given hash key. Use only "<", ">", "<=", ">=" as needed, or else use sprintf on the data value to get it into the same precision as the hash key, then use "eq" (or "gt", "ge", "le", "lt").

I'm still scratching my head about the "0, 0.1, 0.3, ..." series -- that jump from 30 to 1000 seems odd.