I have written several subs that have optional parameters but have always put the at the end.
<pseudo-code>

sub foo {
    my ($p1, $p2, $p3, $p4, p5) = @_;
    do { #something } if $p4;
    do { #something } if $p5;
    #etc
}
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</pseudo-code>
Then you can call your sub with 3,4,5 (or any number) of parameters

or

you can call your sub like:

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

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