my @array = <$fh>;
for (my $i=0; $i<@array; ++$i) {
   my $line = $array[$i];
   ...
   if (...) {
      defined( my $next_line = $array[++$i] )
         or die;
      ...
   }
   ...
}

##</code><code>##

while (my $line = <$fh>) {
   ...
   if (...) {
      defined( my $next_line = <$fh> )
         or die;
      ...
   }
   ...
}