$ perl -MData::Dumper -e 'sub t{ $_+=1 for @_}; my %h = (1 => 2, 3 => 4); t(%h); print Dumper \%h'

##</code><code>##

$VAR1 = {
          '1' => 3,
          '3' => 5
        };

##</code><code>##

$VAR1 = {
          '2' => 3,
          '4' => 5
        };