in reply to data type discovery inside sub: how?

Missing semicolon on that line:
$sysChassisID = getSnmpInfo (@sysChassisIDOID)

which isn't quite right in your context anyways. You are passing a list consiting of the array elements into the sub, but you seem to want to pass a reference:

$sysChassisID = getSnmpInfo (\@sysChassisIDOID);

--shmem

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}