use warnings;
use strict;

sub foo {
    my $bar = shift;
    print "original foo($bar)\n";
}

if (my $pid = fork()) {
    close PARENT;

    foo('parent');

    close CHILD;
    waitpid($pid, 0);
} else {
    die "cannot fork: $!" unless defined $pid;
    close CHILD;

    # overload the function
    no warnings 'redefine';
    sub foo {
        my $bar = shift;
        print "modified foo($bar)\n";
    }

    foo('child');

    close PARENT;
    exit(0);
}

foo('the end');