open(VIRTUALPRN, "| perl print.pl");

##</code><code>##

print VIRTUALPRN "foobar\n"

##</code><code>##

select VIRTUALPRN;  # make this the "default" output handle
$|++;  # disable output buffering on this handle

##</code><code>##

use FileHandle;
...
my $VirtualPrnFH = new FileHandle;
$VirtualPrnFH->open( "| perl print.pl" );
$VirtualPrnFH->autoflush(1);  # turn off buffering
...
print $VirtualPrnFH "foobar\n";
...