use strict;
use warnings;
use 5.010;

my $pid = fork;

if (!defined $pid) {
    die "Couldn't fork: $!";
}

if ($pid) {  #then in parent

    local $SIG{CHLD} = sub {say 'caught it';};
    say 'hello world';
}
else { #then in child

   die; 
}

--output:--
hello world


##</code><code>##

caught it
hello world

#or

hello world
caught it