This is a really common way to do things in Perl OO, but need not be limited to that. It gives you the opportunity to allow defaults etc.

Something I do a lot (in OO) is this:

my %defaults = ( cat => 1,
         dog => 2 );
my @allowed = keys(%defaults); # you might allow others in here too.

sub new
{
    my $class = shift;
    my %args = (%defaults, @_);

    my @hash{@allowed} = @args{@allowed};  # ignore arguments
                           # that we're not expecting.
    # do any specific checking here.

    my $self = \%hash;
    return bless ($self, $class);
}
[download]

The key point here is that this makes it easy for the user to pass the same set of arguments to a subroutine/constructor and for the class to only use the ones it wants. What is more, this method of passing arguments means that the user doesn't have to remember what order things go in.

That's probably the biggest advantage of this method passing, not for prototyping purposes but that it saves the user having to remember argument order, especially if you have "optional" arguments.

Defaults can be done when you use shift too.

sub mysub
{
    my $cat = shift || 1;
    # etc
}
[download]

By the way, putting the defaults hash outside of your subroutine means that you need build it only once (and you can modify it during the program if needed, but this can be a bad thing too).