Is there any way to prevent that "fork"?

AFAIK, no. See exec, system, and qx.

i need to run many externals commands from my Perl script and capture both return-code and STDOUT. These commands must run under a shell...

Be aware of The problem of "the" default shell. I also wrote about running external commands here. In this case I'd suggest something like IPC::System::Simple (note it doesn't capture STDERR, for that I'd suggest a different module):

use IPC::System::Simple qw/capturex EXIT_ANY/;
my $out = capturex '/bin/bash', '-c', 'echo foo';

# or, if you don't want it to die on nonzero exit:
my $out2 = capturex EXIT_ANY, '/bin/bash', '-c', 'echo bar && false';
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i am looking for the "cheapest way" in terms of CPU and Memory load...

Without more information, this smells like premature optimization. What OS? What commands are you running; could they perhaps be replaced by Perl code? How many commands per second are you running? How fast do you need it to run, and have you written a prototype and measured its performance to see if it's good enough or not?

Thanks everybody.... i was just looking for to prevent the "fork" but i am afraid it is un-avoidable...

Looking to avoid the fork indicates you don’t quite have a handle on Unix process design. It is impossible to start another process on Unix without first forking a program; the child process then calls exec to become the other program of your choice. there is no equivalent to Win32 CreateProcess (though you could use inter-process communication to ask some other daemon to fork/exec for you)

What it sounds like you actually want to avoid is the call to “wait” that blocks the parent process until the child has ended. The module IPC::Run is a good way to run asynchronous processes and capture their output.

As far as I remember IPC::Run can be used for external commands without calling a subshell. You may see Re^3: Reading output of external program without Shell, a node where haukex was involved. Regards, Karl