Work directly on @_ instead of making a copy by the my-operator:

use strict;
use Data::Dumper;
my %bits;
$bits{'zero'}='start element';
&loader('something',\%bits);
print 'Result:'.Dumper(\%bits);

sub loader{
 # loads values into hash..
 my $tail = shift;

 print 'In sub:'.Dumper($_[0]);

 $_[0]->{'one'}='thing_'.$tail;
 $_[0]->{'two'}='thong_'.$tail; 
 
 print 'After add:'.Dumper($_[0]);
}
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ciao, Veit Wehner