I changed the code to

use DBI;
use FileHandle;
use rf;
use File::Spec;
use IO::Socket;
use POSIX qw(setsid);

sub get_msg_id_and_type;
sub REAP {
    1 until (-1 == waitpid(-1, WNOHANG));
    $SIG{CHLD} = \&REAP;
}
$SIG{CHLD} = \&REAP;
#$devnull = File::Spec->devnull();
open STDIN, '>/dev/null'  || die "Can't read /dev/null: $!";
#open STDOUT, '>/dev/null'  || die "Can't write to $prout: $!";
open STDOUT, File::Spec->devnull()   ||  die "Can't write to $prout: $
+!";
open STDERR, '>>/dev/null' || die "Can't write to $prerr: $!";
defined(my $pid = fork)   || die "Can't fork: $!";
exit if $pid;
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and it works. Is it because the /dev/null behaviour is different on HP-UX and LINUX?