I cannot find a single correct statement in your post, even after careful testing of each point you make, so I have to assume you are a poseur who intends to disrupt and disinform the audience.

Update: For example, if ($child) will likely be undef, so what? it doesn't yield a warning. Its easy enough to test my code by putting a print statement for each section. If you comment out the exit, a print statement after the if block gets executed twice as I advertised. If you put sleep 12 in the "job processing section" it will execute the loop a couple of times for clarity.

Update+: 

#!/usr/bin/perl
use POSIX ":sys_wait_h";
my $child = fork;
if ($child) { # parent case
    my $res;
    while (not ($res = waitpid($child, WNOHANG))) {
        sleep 5;

        # check file list here
        # ...
        print "checking file list\n";
    }
    if ($res == -1) {
        warn "fatal child error\n";
        exit;
    }
}
else { # child case
    # insert job processing here
    print "job processing\n";
    sleep 12;
    exit; # comment this out to see how many times the final print occ
+urs if you don't exit."
# child must explicitly exit otherwise both would continue executing t
+his
}
print "Past the if\n";

[download]
update++: produces this with exit in place 
job processing
checking file list
checking file list
checking file list
Past the if

[download]
and this with exit removed: 
job processing
checking file list
checking file list
Past the if
checking file list
Past the if

[download]

One world, one people

In reply to Re^3: Doing every X seconds by anonymized user 468275
in thread Doing every X seconds by dideod.yang

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