Actually the only guarantee for writing is 1 byte (supposing you don't get an error) so if you try to write more you might block, that's why if you don't want to be blocked you need to go all the way to non-blocking handles (which is never easy as you have to deal with partial writes).

If I need to go non-blocking I usually use a variation on Lincoln's Stein IO::SessionData and IO::SessionSet. Sadly they are packaged with SOAP::Lite, and having "godzilla" as a prerequesite is no fun ;) IO::Multiplex is also interesting.

Interesting. I thought syswrite would only write as much as it could without blocking, but my test (on FreeBSD 6.1) shows otherwise.

Test:

use strict;
use warnings;

use IO::Handle qw( );
use IO::Select qw( );

{
   pipe(my $fh_r, my $fh_w)
      or die("Unable to create pipe: $!\n");

   $fh_w->autoflush(1);

   my $sel_w = IO::Select->new($fh_w);

   {
      print("Filling up the pipe...\n");

      my $buf = 'x';

      for (;;) {
         my @ready_w = $sel_w->can_write(0.5);

         # Pipe is full!
         last if not @ready_w;

         die if @ready_w != 1;
         die if $ready_w[0] != $fh_w;

         my $rv = syswrite($fh_w, $buf, length($buf));

         if (not defined $rv) {
            die("Unable to write to the pipe: $!\n");
         }

         if (not $rv) {
            die("Unable to write to the pipe: 0 bytes written??\n");
         }
      }
   }

   {
      print("Opening up one byte in the pipe...\n");

      my $buf = '';

      my $rv = sysread($fh_r, $buf, 1);

      if (not defined $rv) {
         die("Unable to read from the pipe: $!\n");
      }

      if (not $rv) {
         die("Unable to read from the pipe: 0 bytes read??\n");
      }
   }

   {
      print("Make sure it's safe to write to the pipe...\n");

      my @ready_w = $sel_w->can_write();

      die if @ready_w != 1;
      die if $ready_w[0] != $fh_w;
   }

   {
      print("Writing more chars to the pipe than the pipe can handle..
+.\n");

      my $buf = 'x' x (1024*1024);

      my $rv = syswrite($fh_w, $buf, length($buf));

      if (not defined $rv) {
         die("Unable to write to the pipe [2]: $!\n");
      }

      if (not $rv) {
         die("Unable to write to the pipe [2]: 0 bytes written??\n");
      }

      printf("Wrote %d bytes to the pipe\n", $rv);
   }

   close($fh_w);
   close($fh_r);
}
[download]

Output:

Filling up the pipe...
Opening up one byte in the pipe...
Make sure it's safe to write to the pipe...
Writing more chars to the pipe than the pipe can handle...
[blocks]
[download]

That's unfortunate.