system always returns 0 on successful launch. I recommend you do all the pid-saving in a wrapper shellscript (or Perlscript) around your child program:

#!/usr/bin/perl -w
use strict;

my $pid = $$;
my ($IPaddress) = @ARGV;

# Save our PID into the pidfile:
my $pidfile = "/home/ember/devices/$Macaddress/$pid.pid";
open (FILE, "> $pidfile") or die "Can't create file $pidfile: $!\n"; 
print FILE "$pidfile\n";
close FILE;

# and replace ourselves with the real program, which gets the same PID
+ as we had:
exec 'other.pl', $IPaddress;
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