There's a few problems with your code:

• Instead of "main::date" try "$date" - you shouldn't need to specify main:: unless you specified a different package.
• You are returning a value (the literal string "main::date"), but thankfully, you're not using it.
• You are splitting on an uninitialised variable ($_), which you would have known if you had warnings turned on. Use $_[0] instead.
• You can change all the splits into one split:

  my ($year, $month, $day, $hour, $min, $sec) = 
      split /[- :]/, $_[0];
  [download]

• You don't need to modify the global - just return the value you need:

  
  my $date = dateconverter($date_begin);
  print $date, "\n";
  
  sub dateconverter {
    my @bits = split /[- :]/, $_[0];
    return join ".", @bits[0,1,2];
  }
  [download]

-- Dan