sub bad_average {
   my $acc = 0;
   $acc += $_ for @_;
   return $acc / @_;
}

sub better_average {
   my $acc = 0;
   $acc += $_/@_ for @_;
   return $acc;
}

##</code><code>##

bad:     if ($x == $y)
abs tol: if (abs($x - $y) < $abs_tol)
rel tol: if (abs($x - $y) < abs($x) * $rel_tol)

##</code><code>##

bad:  for (my $x=0; $x<100; $x+=0.1) { ... }
good: for (0..99) { my $x = $_/10; ... }