I have a simple script I want to daemonize, but it won't and still hanging in the terminal.

#!/usr/bin/perl -T
use strict;
use warnings;
use POSIX 'setsid';


$ENV{'PATH'} = '/bin:/usr/bin:/usr/sbin';
delete @ENV{'IFS', 'CDPATH', 'ENV', 'BASH_ENV'};

my $p0f;
my $pid;

open($p0f, "p0f -l 'tcp dst port 25' 2>&1 | ") or die "Can't fork: $!"
+;

daemonize();

while(<$p0f>) {
  print if /Linux/;
}


sub daemonize {
    chdir '/'  or die "Can't chdir to /: $!";
    open STDIN, '/dev/null' or die "Can't read /dev/null: $!";
    open STDOUT, '>/dev/null' or die "Can't write to /dev/null: $!";
    defined(my $pid = fork) or die "Can't fork: $!";
    exit if $pid;
    setsid or die "Can’t start a new session: $!";
    open STDERR, '>&STDOUT' or die "Can't dup stdout: $!";
}
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the script is not backgrounded and parent process id still showing in ps command

I suspect it might be open($p0f, "p0f -l 'tcp dst port 25' 2>&1 | ") or die "Can't fork: $!" STDERR redirecting thing, anybody know how to detach STDIN/STDOUT/STDERR properly while I can still do the redirecting thing.

If I remove the daemonize sub and run the script as sample.pl & at terminal, it will be backgrounded properly. I would like daemonize sub to do the same thing as '&'.