If you wanted to avoid using a hash of named arguments for whatever reason, but wanted to avoid typing

my $argN="default";
$argN=shift if @_;
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repeatedly, perhaps the following would be useful :

&foo (1,2,3);

sub foo {
    my @defaults= qw (foo bar baz quux quuz boo);
    for (1.. @_) {shift @defaults};
    @_= (@_, @defaults);
    ($a1, $a2, $a3, $a4, $a5, $a6) = @_;    
    print join "\n",$a1, $a2, $a3, $a4, $a5, $a6;
}
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This doesn't check that a user's supplied too many parameters, but that shouldn't be too hard to add in.
Of course, as mentioned elsewhere, this is all moot once // comes into play, but it's an interesting mental exercise.

An additional note : this assumes that parameters are given in "least optional" to "most optional"; if you have 6 params and only #2 is optional, this won't do it for you.