A third way is to call pipe before fork to set up a reader and writer handle. You'll want to set the writer to autoflush, and to close the unwanted end of the pipe in each process.

That is low level, so is very flexible and powerful. Writing C in Perl is not always a bad thing.

Here's how to set up the child as a coprocess, taking STDIN from the parent, and writing back STDOUT to the parent. That needs two pipes, one each direction. 

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;

[download]
Untested. Will fail if foo is naive about file descriptors for STDIN and STDOUT.

After Compline,
Zaxo

In reply to Re: child processes in fork by Zaxo
in thread child processes in fork by Anonymous Monk

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.