I'm not speaking from a personal knowledge of the Perl source code, but I have always been under the assumption that open calls with leading or trailing pipes do a fork/exec to start the subprocess.

If the exec fails (e.g., can't find the command) then the open returns a failure (in the parent process). If the exec succeeds, then the open returns success (in the parent process) and your program can start reading or writing to the sub-process.

You may get more clues about behavior you're seeing from the fork and exec documentation.

    perldoc -f fork
    perldoc -f exec
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(All of my posts should be implicitly prefaced with the fact that I only speak of Unix/Linux/Solaris/... In my world I tend to forget that there are environments like Windows/OS2/VMS...)

-- Eric Hammond