The main drawback I see with your code is that it copies elements from past the end of the array.

Here's a solution where the source index is the loop variable, and the destination follows behind it:

my $p = 0;
for (my $q = 0; $q < @{$mon_obj->{'FILE_DATA'}}; ++$q) {

  if (defined $mon_obj->{'FILE_DATA'}->[$q]) {

    $mon_obj->{'FILE_DATA'}->[$p] = $mon_obj->{'FILE_DATA'}->[$q]
      if $p != $q;

    $p++;

  }
}

splice @{$mon_obj->{'FILE_DATA'}}, $p;
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