To send a complete file from disk, without applying any modifications first, instead of:

my $filename = "/tmp/foo";
my $fh = Apache::gensym( );  # generate a new filehandle
open $fh, $filename or return NOT_FOUND;
print <$fh>;
close $fh;

it's better to write:

my $filename = "/tmp/foo";
my $fh = Apache::gensym( );  # generate a new filehandle
open $fh, $filename or return NOT_FOUND;
$r->send_fd($fh);
close $fh;

The former implementation uses more memory and it's slower, because it creates a temporary variable to read the data in and then print it out. The latter uses optimized C code to read the file and send it to the client.