I'm going to broaden the topic to positional arguments for future reference. For positional arugments, the equivalent of exists that davidrw suggested for hashes would be to check the size of @_.

sub my_sub {
   my ($arg1, $arg2, $opt_arg1, $opt_arg2) = @_;

   $opt_arg1 = default if @_ < 3;
   $opt_arg2 = default if @_ < 4;

   ...
}
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That allows all false values to be specified, including undef.