my ($reader,$writer,$cpid);
{
    pipe $reader, local *STDOUT or die $!;
    pipe local(*STDIN), $writer or die $!;

    $cpid = fork;
    defined $cpid or die $!;
    last if $cpid;

    # child
    close $reader or die $!;
    close $writer or die $!;
    $| = 1;
    exec '/path/to/foo', @foo_args;
    die 'Failed to launch foo: ', $!;
}

# parent
select((select($writer), $| = 1)[0]); # see 'perldoc -f select'

# builtin 4-arg select or IO::Select may be handy here
# . . .
waitpid $cpid;