There are couple of different ways, take a look at perlipc.

For example. Parent:

use strict;
use warnings;
    
my $pid = open(KID_TO_READ, "-|", "perl -w b.pl");
while (<KID_TO_READ>) {
    print "from parent: $_";
}
[download]

Child:

use strict;
use warnings;

print "Hello\n";
print "World\n";
[download]

This gives you:

from parent: Hello
from parent: World
[download]

However this does not handle STDERR, one way to resolve it is to do this in child:

use strict;
use warnings;

open STDERR, '>&STDOUT';

print "Hello\n";
print STDERR "World\n";
[download]