Depending on your application, you can trade memory for speed. So if all you are really interested in is the information you extract above, you can pre-calculate and store the results. Or even just cache results like this if you will call repeatably...

my %maxIntesityCache;
sub getIntensity {
  my $m = shift;
  # Do we know this already?
  return $maxIntesityCache{$m}
    if defined $maxIntesityCache{$m};
  ...as for your function, except you can probably change
  the core to remove the exists test and/or one of the other
  suggestions in this thread...
  # Remember this for later
  $maxIntensityCache{$m} = $intensity;
  return $intensity;
}
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Precalculation may give you a win because you can run through the keys once, keeping a window from -0.3 to +0.3 open on your key at all times.

Another thought...if all your numeric work is to precision '0.1' you might get a nice speedup by having your key as 'mass*10' so you are only storing integers. You can then:

• avoid the sprintf you are using to control precision issues, which must hurt you now I come to think of it
• you can switch perl into 'use integer' mode which might help too.