(Random hint, you're probably writing to STDOUT, so replace OUTPUT_HANDLE with STDOUT in your snipppet.)

Yes... almost three years later, your post helped a struggling perl scripter... Just wanted to say thanks for the suffering from buffering! Helped a lot. :-j

Try

my $old_fh = select(STDOUT);
$| = 1;
select($old_fh);
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• The first select changes the default output filehandle to STDOUT and returns the current default output filehandle to be saved in $old_fh.
• $|=1 turns on autoflushing on the current output filehandle
• select($old_fh) restores the original output filehandle

Generally, if you just want to turn on autoflushing on STDOUT, a simple $|=1 will do.

This worked nicely:

my $old_fh = select(STDOUT);
$| = 1;
select($old_fh);
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Thanks!

Don't really understand that code

I use the more readable

use IO::Handle qw( );  # For autoflush
OUTPUT_HANDLE->autoflush(1);
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If you want to flush without turning on autoflush,

use IO::Handle qw( );  # For flush
OUTPUT_HANDLE->flush();
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(You only need to load IO::Handle once in your program, not everytime autoflush or flush are called.)

Is it possible with IO::Handle or otherwise to make printing to terminal fully buffered? And then do the flushing manually with $io->flush when I want the output.

I once wanted to do this to see if it was a noticeable optimization, but it was not important enough to spend time on. It is still not important but I would like to know anyway if someone happens to know.

->autoflush(1) will flush after every print/syswrite/etc.
->autoflush(0) will flush on demand (->flush), when the buffer is full (all handles) and when "\n" is encountered (when -t HANDLE only).

If you want something else, you'll need a tied handle or your own print function.

Good question.  With older Perls (pre-5.8) you could do

use IO::Handle '_IOFBF';

my $buf;
# set fully buffered, with a buffer size of 1024 bytes
STDOUT->setvbuf($buf, _IOFBF, 1024);
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but these days, setvbuf/setbuf are no longer available unless perl is compiled to explicitly not use perlio.

I'd like to know myself whether there is a substitute for setvbuf (without writing your own XS), as on rare occasions there might be valid uses for being able to control buffering mode or buffer size with Perl. (The function doesn't seem to be POSIX, so use POSIX; wouldn't help, AFAIK...)

If you run the program from the command line, add a newline (\n) to the end of your print statements. STDOUT uses line buffering when connected to an active terminal.