Here's how to do that in perl without any interference from the shell:

$SIG{CHLD} = 'IGNORE';
{
    my $cpid = fork;
    defined $cpid or warn 'Could not fork: ', $! and last;
    $cpid and last;
    # child
    open STDERR, '>>', $errorfile || '/dev/null'
        or die $!;
    open STDOUT, '>', $outputfile || '/dev/null'
        or die $!;
    open STDIN, '<', $inputfile || '/dev/null'
        or die $!;
    $SIG{HUP} = sub {};
    exec "/mevs/$version/bin/mevs",
         "/mevs/$version/data/config_data/mevs2.cfg",
         'online';
    die $!;
}
# parent
[download]

That does about what nohup does. That can be further refined by imitating how a daemon starts with POSIX::setsid() to declare the child a session leader and remove itself from a controlling terminal.

Shell-like access to a background process is neither necessary or desirable in perl. We have finer weapons than that.