I don't think it's on the system level, but it's perl that retries. I don't know exactly which functions Perl executes on backticks, but it's probably going to open some pipe. And here's a snippet from the function Perl_my_popen in util.c: 
    while ((pid = PerlProc_fork()) < 0) {
        if (errno != EAGAIN) {
            PerlLIO_close(p[This]);
            PerlLIO_close(p[that]);
            if (did_pipes) {
                PerlLIO_close(pp[0]);
                PerlLIO_close(pp[1]);
            }
            if (!doexec)
                Perl_croak(aTHX_ "Can't fork");
            return NULL;
        }
        sleep(5);
    }

[download]
As you can see, if the fork fails with an EAGAIN, perl sleeps for 5 seconds and tries again.

A similar loop is found in Perl_my_popen_list in the same file.

In reply to Re^4: Ulimit makes program hang (bug?) by JavaFan
in thread Ulimit makes program hang by memo2005

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