I generally prefer a hash ref, because I find it easier to work with.

sub hash 
{
    my %in = @_;
    . . . 
}

sub hashref 
{
    my $in = shift;
    my %in = %{ $in };
    . . . 
}

hash( foo => 'bar' );
hashref({ foo => 'bar' });
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At first glance, it looks like there is slightly more code to do a hashref compared to a plain hash. However, IIRC, mis-formatting the hash in the subroutine call will be a run-time error, whereas mis-formatting the hashref will be compile-time. IMHO, if you get to choose, compile-time errors are better than run-time errors. Additionally, hashrefs let you sprinkle in a few positional params, if you so choose.

sub hashref_and_positional 
{
    my $param1 = shift;
    my $param2 = shift;
    my $in     = shift;
    my %in     = %{ $in };

    . . . 
}

hashref_and_positional('foo', 'bar', { baz => 'blah' });
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----
I wanted to explore how Perl's closures can be manipulated, and ended up creating an object system by accident.
-- Schemer

Note: All code is untested, unless otherwise stated