After you do this:

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

... and before you do this:

print VIRTUALPRN "foobar\n"
[download]

you could try this:

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

Or, more elegantly:

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

If you're only writing a small amount of content to that process, the default buffering would withhold output to the process until the file handle closes, which you could do explicitely in the parent process (rather than just exiting), but it's better to turn off buffering.

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