$cmd="$scriptsdir/process_tar.pl $file >> /tmp/testlog &";            
+          
+                  
$pid=open(PID,"|$cmd");
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you are opening a pipe to a shell which then runs perl. The shell and the perl process don't have the same PID:

#!/usr/bin/perl
# file pid.pl
sleep 1; print "$0 PID: $$\n";
[download]

#!/usr/bin/perl

my $cmd = 'echo my PID is $$ && ./pid.pl';
my $pid = open PID, "| $cmd"
    or die "Can't run ./pid.pl: $!\n";

print "PID returned by open: $pid\n";
sleep 2;
__END__
my PID is 4465
PID returned by open: 4465
./pid.pl PID : 4466
[download]

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}