my $output_file = 'output_0000';
my $file_size = 0;
open(STDOUT,>,$output_file)
  or die "Error opening $output_file for writing, ";
while (my $line = <>)
{
   do_something($line);
   if ($file_size > 50000)
   {
      close STDOUT
        or die "Error closing $output_file, ";
      $output_file++; # magic increment
      open(STDOUT,>,$output_file)
        or die "Error opening $output_file for writing, ";
      $file_size = 0;
   }
   print $line;
}