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;
}