Buffering is on by default. It can be turned off. The following will cause DAT to be flushed after every print:

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

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

...
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If you don't want it to be automatic, use

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

flush(*DAT);
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Alternative, IO::Handle provides methods flush, autoflush and printflush, so you could use child class IO::File to open the file:

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.
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By the way, you see the data when it's sent to the screen because STDOUT is buffered in a special way: Printing a newline causes a flush, even if autoflushing is off for that handle.