use strict;
use warnings;

my %hash = (1,'a',2,'b',3,'c');
print $hash{2};
mysub(\%hash);
print $hash{2};

sub mysub {
    my $p = shift;
    print $p->{2};
    $p->{2} = 'd';
}

##</code><code>##

use strict;
use warnings;

my %hash = (1,'a',2,'b',3,'c');
mysub(\%hash);

sub mysub {
    my %h = %{shift()};
    print $h{2};
}