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  1. or download this
    my $sum = 0;
$sum += (-1)**($_+1) * $_ for 1 .. 1000;
print "$sum\n";
  2. or download this
    my $sum = 0;
my $sign = 0;
for my $i (1 .. 1000) {
...
    $sign = !$sign;
}
print $sum;
  3. or download this
    my $sum = 0;
my $sign = 0;
for my $i (1 .. 1000) {
    $sum += ($sign++ % 2 ? -1 : 1) * $i;
}
print $sum;
  4. or download this
    my $sum = 0.5*(1 + 999)*( (999-1)/2 + 1 ) - 0.5*(2 + 1000)*( (1000-2)/
+2 + 1 );
print $sum;