sub foo {
    my ($p1, $p2, $p3, $p4, p5) = @_;
    do { #something } if $p4;
    do { #something } if $p5;
    #etc
}


##</code><code>##

prog(1,"","",2,3)

##</code><code>##

prog(1,undef,undef,2,3)