open(DAT, '>', ...) || die(...);

{
   my $old_sel = select(DAT);
   $| = 1;  # Auto-flush.
   select($old_sel);
}

...

##</code><code>##

sub flush {
   my $h = select($_[0]); my $af=$|; $|=1; $|=$af; select($h);
}

flush(*DAT);

##</code><code>##

my $dat = IO::File->new($filename, 'w')
or die(...);

$dat->autoflush(1);
print $dat ...;          # Auto-flushed.
$dat->print(...);        # Auto-flushed.

$dat->autoflush(0);
print $dat ...;          # Not flushed.
$dat->print(...);        # Not flushed.
$dat->flush();           # Flushed.

$dat->printflush(...);   # Flushed.