my $i = 0;
while (<FILE>) {
  next if ++$i % 2;
  print;
}

##</code><code>##

while (<FILE>) {
  next if $. % 2;
  print;
}

##</code><code>##

my $toggle = 0;
while (<FILE>) {
   next if $toggle ^= 1;
   print;
}

##</code><code>##

for (;;) {
   last if not defined (my $line1 = <FILE>);
   last if not defined (my $line2 = <FILE>);

   next if not $line1 =~ /red line/;

   print($line1, $line2);
}