A more specific and illustrative example is $small+$huge1-$huge2 where ($small+$huge1)-$huge2 is easily much less accurate than $small+($huge1-$huge2) (where parens imply order of evaluation, which isn't always the case).
For example:
#!/usr/bin/perl -lw
use strict;
my $h1= my $h2= 1e20;
my $s= 1.2345;
print $s+$h1-$h2;
print $s+($h1-$h2);
__END__
0
1.2345
(I'd actually be surprised to see an example where the difference you proposed, ($a+$b+$c)/$d vs $a/$d+$b/$d+$c/$d, made more than a couple of least-significant bits of difference in the result -- though that would be enough to thwart the use of ==, of course.)