It's not clear to me why you are using fork here but that certainly seems to be the root of your troubles. The doc for alarm says:

If you want to use "alarm" to time out a system call you need to use an "eval"/"die" pair.

... but you haven't done that in 6.fork.pl and therefore the ping processes are not reaped which is presumably why you see high icmp_seq counts on subsequent runs. Why use alarm at all instead of ping's built-in timeout?

Here are 2 examples with neither fork nor alarm. First, using system to execute the ping command:

#!/usr/bin/env perl 

use strict;
use warnings;

for my $dest ('www.google.com', 'www.perlmonks.org', 'microsoft.com', 
+'foo.bar') { 
    system ("ping -nqc1 -w 3 -W 3 $dest > /tmp/ping.log") and
        print "### $dest is unreachable\n";
}
[download]

and now the same with Net::Ping:

#!/usr/bin/env perl 

use strict;
use warnings;
use Net::Ping;

my $pinger = Net::Ping->new ('icmp', 3);
for my $dest ('www.google.com', 'www.perlmonks.org', 'microsoft.com', 
+'foo.bar') {      
    $pinger->ping ($dest) or
        print "### $dest is unreachable\n";
}
$pinger->close;
[download]

Note that in both cases, microsoft.com is unreachable because it doesn't respond to pings and foo.bar is unreachable because it doesn't exist. The magic number 3 is the timeout to enforce in each case. As per the documentation the second script must be run as root to use ICMP pings.

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