in reply to Perl crashing with Parallel::ForkManager and WWW::Mechanize

2. For understanding purpose, I added a print statement right after the finish method of fork manager, however it does not print that.

This is because the child process gets terminated earlier by $pm->finish();, and parent process skips the code because of the next statement in $pm->start() and next;.

4. I have not understood why, wait_all_children method is place outside the while or the for loop though (as observed in the documentation as well), since all the processing is taking place inside the loop.

As it's said in the documentation,

use $pm->start to do the fork. $pm returns 0 for the child process, and child pid for the parent process (see also "fork()" in perlfunc(1p)). The "and next" skips the internal loop in the parent process. NOTE: $pm->start dies if the fork fails. $pm->finish terminates the child process (assuming a fork was done in the "start").
$pm->start acts like fork. In the point of the program where fork is called a new process is created (called child process), the other process is called "parent". fork returns its PID in the parent process; any integer means "true" to Perl, so fork and next lets parent process start next iteration of loop. fork returns 0 to the child process; 0 means "false" to Perl, so fork and next does not let the child process skip the loop code.

So, the parent process creates all the children then waits for them to finish outside the loop.

Sorry if my advice was wrong.