# your code
my $columnsum += $_->[$colnum] for @data;

# is effectively the same as
for (@data) {
   my $columnsum += $_->[$colnum]
}

# so as expected $columnsum is effectively localised to that block (line).

my $i += $_ for 1..3;
print "localised (1) \$i='$i'\n";
for (1..3) { my $i += $_ }
print "localised (2) \$i='$i'\n";
$i += $_ for 1..3;
print "not localised (1) \$i='$i'\n";
for (1..3) { $i += $_ }
print "not localised (2) \$i='$i'\n";
print "Oops, forgot to zero a widely scoped var!";