if you are restricted to positional parameters, what about this ...
sub defaults {
my $d1= ( @_ ? shift : 1 );
my $d2= ( @_ ? shift : 2 );
my $d3= ( @_ ? shift : 3 );
print("$d1 $d2 $d3\n");
}
defaults();
defaults(0);
defaults(0, 0);
defaults(0, 0, 0);
__DATA__
1 2 3
0 2 3
0 0 3
0 0 0
UPDATE: precedence rules allow to omit the parens.
UPDATE from the example you gave I assumed that you don't plan to pass undef in the middle of parameters and expect this to be checked... otherwise all these solutions are too simplified and you have to check explicitly with defined. But in this case I really recommend using named parameters!
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