using defined on fork

[Anonymous Monk]

I don't understand this following code:

defined(my $pid=fork) or die $!;
unless($pid){
exec 'date';
die $!;
}
waitpid($pid,0);

when using fork,the fork function returns pid, which is non-zero number and true value. I don't know why I should use defined function in here. But without defined, this code just immediately die

[Corion]

The return values of fork are listed in the documentation for fork. Fork can return three values - 0 for the parent, nonzero for the child and undef if the fork fails.

So, the defined check is there to check if the fork was successful.