use POSIX 'setsid';
$SIG{HUP} = 'IGNORE';
#chdir('/')  # Not required in your case; maybe not desired.
#    or  die "Can't cd /: $!\n";
for( 1,0 ) {
    my $pid = fork();
    die "Can't fork: $!\n"
        if  ! defined $pid;
    exit
        if  $pid;
    if(  $_  &&  -1 == setsid()  ) {
        die "Can't setsid: $!\n";
    }
}
open STDIN,  '<', '/dev/null' or die "STDIN: $!\n";
open STDOUT, '>', '/dev/null' or die "STDOUT: $!\n";
open STDERR, '>&', \*STDOUT   or die "STDERR: $!\n";