\@{$ref} is the same as $ref, because @{$ref} accesses the array that $ref points to, and adding a \ gets the same reference.

While accessing @_ inside a function does work, I think you should put your arguments in variable (which should be done when @_ isn't a list of similar things but each argument has its own meaning. So:

sub repl_mongo{
  my ($ref, $depth) = @_;
  return unless $depth;
  if (ref($ref) eq 'ARRAY') {
    ...
  }
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foreach my $item (keys %{$_[0]}){ repl_mongo($key,$_[1]-1 ); } The loop variable is $item, but you access $key. Do you have strict on? It should have told you about that.

And for the date conversion, after you have found that a string starts with ISODate (that's index exactly 0 by the way) you can use substr to extract the date part (which will always be at the same positions, starting from the 8th character).