my $pid = open my $fh, "| $some_program"
    or die "Can't run '$some_program': $!\n";

##</code><code>##

#!/usr/bin/perl -w

use strict;

open(O,'>', "$$.tmp");
select O; $| = 1; # make output unbuffered
print STDOUT "file is $$.tmp\n";

$SIG{'CHLD'} = \&reap;
if ( (my $pid = fork()) == 0 ) {
    sleep 1;
    print "child ($$) here\n";
    close O;
    exit 0;
} else { 
    print "parent ($$) here\n";
    sleep 2;
    print "parent ($$) waking up..\n";
}   

sub reap {
    my $pid = wait;
    print "reaped PID $pid\n";
}
__END__
file is 4782.tmp

##</code><code>##

$ cat 4782.tmp
parent (4782) here
child (4783) here
reaped PID 4783
parent (4782) waking up..