The most portable way of doing this that I can think of is to use fork followed by exec. Consider the following:

sub run_in_bg {
    my $pid = fork;
    die "Can't fork a new process" unless defined $pid;

    # child gets PID 0
    if ($pid == 0) {
        exec(@_) || die "Can't exec $_[0]";
    }

    # in case you wanted to use waitpid on it
    return $pid;
}
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You'd call this the same way you'd call system, except now it forks a new process before running the first arg.