you should de-reference using e.g. print $Mdevices[$k]->[0]

$Mdevices[$k]->[0] is functionally the same as $Mdevices[$k][0] (what the OP had).  You only need the arrow if it's at the 'first' level (i.e. $Mdevices->[0] is not the same as $Mdevices[0] — here, the parser needs some way to disambiguate). In case it isn't ambiguous (between brackets/braces), the arrow is optional.