I'm not sure fork is the right tool here. I recently came across an easy to use module to put things in the background, Proc::Background that may help (cross-platform even).

use warnings;
use strict;

use Proc::Background;

my @cmd = qw(perl sleep.pl);

my $proc = Proc::Background->new(@cmd);
my $pid = $proc->pid;

print "*** $pid ***\n";
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Output showing parent exits while proc still running:

~/scratch$ perl proc.pl 
*** 3489 ***

~/scratch$ ps ax | grep sleep
 3489 pts/1    S      0:00 perl sleep.pl
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