Child processes inherit their parent's file handles. So, if you want to redirect STDOUT to a file in both the parent and the child, you can do something like this:

close STDOUT;
open STDOUT, ">out" or die;

my $pid = fork();
die "" if !defined $pid;
exit 0 if $pid; # parent exits

print "Message!\n"; #child prints to file
[download]

This works as expected and prints to the file, not to STDOUT. You can do something similar by tie()ing the file handle to a trapper class:

tie *STDOUT, TrapClass;

my $pid = fork();
die "" if !defined $pid;
exit 0 if $pid; # parent exits

print "Message!\n"; # child prints to file

package TrapClass;

sub TIEHANDLE {
    my $class = shift;
    open(my $fh, ">out") or die;
    bless { fh => $fh }, $class;
}

sub PRINT {
    my $self = shift;
    my $fh = $self->{fh};
    print $fh @_;
}
[download]

This works as well, printing to the file instead of STDOUT. However, if you replace the print statement by exec() to overload the child process with something else, like /bin/date, it doesn't work anymore, the child process will print to STDOUT, not to the file:

tie *STDOUT, TrapClass;

my $pid = fork();
die "" if !defined $pid;
exit 0 if $pid; # parent exits

exec "/bin/date"; # child prints to STDOUT!!

package TrapClass;

sub TIEHANDLE {
    my $class = shift;
    open(my $fh, ">out") or die;
    bless { fh => $fh }, $class;
}

sub PRINT {
    my $self = shift;
    my $fh = $self->{fh};
    print $fh @_;
}
[download]

This is puzzling, given that exec() works just fine if you redirect the file handle the good-old fashioned way:

close STDOUT;
open STDOUT, ">out" or die;

my $pid = fork();
die "" if !defined $pid;
exit 0 if $pid; # parent exits

exec "/bin/date"; #child prints to file
[download]

Does anyone know what kind of dark magic is going on in tie() to breaks the third case?

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