my $sq = sub { $_[0]**2 };
my $id = sub { $_[0] };
 
sub sum {
  my ($a,$b,$func) =@_;
  $func //= $id;
  my $sum;
  $sum += $func->($_)  for ($a..$b);
  return $sum;
}
 
sub sum_sq{ sum($_[0],$_[1],$sq)}
 
$,=",";
print sum(3,5),sum_sq(3,5);	    # 12,50