I (and many others who don't understand rounding) would greatly appreciate if 
the hits variable could be rounded down to the closest 0.1 (rather than just 
rounding) so as to transform the subject phrase into the much clearer 
Fixing "X-Spam-Status: No, hits=4.9 required=5.0".

I'm suggesting that new_hits = int(old_hits * 10) / 10, where int is defined 
at <http://www.cotse.com/perl/int.html>  Int is not a true rounddown function, 
in that if old_hits is -4.54, new_hits will round up to -4.5 rather than down 
to -4.6, but that's a lot less counter-intuitive than what's there today.  And 
yes, I know that the cited Perl documentation recommends sprintf instead of 
int, but in this case I think it will be much clearer to round down rather 
than just round.

I could be completely wrong, but I believe the fix would be in PerMsgStatus.pm 
to round the hits variable down to the same level of precision by which it 
will be shown to users (rather than rounding to 3 decimal places here and 1 
decimal place in the X-Spam-Status header):

  # Round the hits to 3 decimal places to avoid rounding issues
  # We assume required_hits to be properly rounded already.
  # add 0 to force it back to numeric representation instead of string.
  $self->{hits} = (sprintf "%0.3f", $self->{hits}) + 0;

I believe this should be:

$self->{hits} = int($self->{hits} * 10) / 10

but then I don't really know Perl so I wouldn't take my word for it.  Another 
option would be:

  $self->{hits} = (sprintf "%0.1f", $self->{hits}) + 0;

The only difference is:

perl -e 'print sprintf ("%0.1f\n", 4.950)'
5.0

perl -e 'print (int(4.950*10)/10)'
4.9

I would argue the rounddown functionality of the latter is closer to the 
intent of a required hits function than rounding.

Note that one other way of attacking the underlying issue (which is that the 
numbers used by the program logic are not the ones being presented to the 
user) would be to make the use of sprintf consistent throughout the module so 
that it used, say "%2.1f" in all cases, and not just for display.