You are printing the content of the variable. It's two different variables. If you print the variables directly instead of via Dumper,

my $hash = {};

do_something( $hash );
print Dumper( $hash );

sub do_something
{
    my $hash = shift;

    $hash = { a => 'alpha',
              b => 'beta',
            };

    print Dumper( $hash );
}
[download]

you'll get two different addresses:

HASH(0x3c4a4)HASH(0x3c39c)
[download]

Update: actually, this is also only the contents. Best answer so far to your OP is this by AnomalousMonk