Indeed. The fragment offered can be fixed...

push (@Mdevices, [$UnitID, $fileno]);
for ($k=0; $k<=$#Mdevices; $k++)
  {
    print $Mdevices[$k][0],"\n";
  }
  $k=0;
[download]

...the named @Cdevices is not required.

If the more complete code is along the lines:

  my @Mdevices ;
  my @Cdevices ;
  while (... something ...) {
    ... do something to get $UnitID & $fileno ...

    push (@Cdevices, $UnitID, $fileno);

    ... do some other stuff ...

    push (@Mdevices, \@Cdevices);
  } ;
[download]

then @Mdevices is going to end up with 'n' copies of a reference to @Cdevices, which contains a list of the 'n' $UnitID and $fileno seen.

Whereas the almost identical:

  my @Mdevices ;
  while (... something ...) {
    my @Cdevices ;
    ... do something to get $UnitID & $fileno ...

    push (@Cdevices, $UnitID, $fileno);

    ... do some other stuff ...

    push (@Mdevices, \@Cdevices);
  } ;
[download]

fills @Mdevices with 'n' references to 'n' different annonymous arrays, each containing a $UnitID and $fileno pair.

Obvious, really.

Alternatively:

  my @Mdevices ;
  while (... something ...) {
    ... do something to get $UnitID & $fileno ...

    ... do some other stuff ...

    push (@Mdevices, [$UnitID, $fileno]);
  } ;
[download]

will do the trick -- unless the @Cdevices array is required in the loop.