On Rosetta (https://rosettacode.org/wiki/Continued_fraction#Perl )

there is the perl solution to evaluating Continuous fractions. The code given is: (<===== comments added by me)

sub continued_fraction {
    my ($a, $b, $n) = (@_[0,1], $_[2] // 100);  
#<========= what is $_[2] // 100 ??
 
    $a->() + ($n && $b->() / continued_fraction($a, $b, $n-1));
}
 
printf "&#8730;2  &#8776; %.9f\n", continued_fraction do { my $n; sub 
+{ $n++ ? 2 : 1 } }, sub { 1 };  # <===== Note there is no third arg p
+assed
printf "e   &#8776; %.9f\n", continued_fraction do { my $n; sub { $n++
+ || 2 } }, do { my $n; sub { $n++ || 1 } }; # <===== Note there is no
+ third arg passed
printf "&#960;   &#8776; %.9f\n", continued_fraction do { my $n; sub {
+ $n++ ? 6 : 3 } }, do { my $n; sub { (2*$n++ + 1)**2 } }, 1_000; #<==
+== Here there is a 3rd arg
printf "&#960;/2 &#8776; %.9f\n", continued_fraction do { my $n; sub {
+ 1/($n++ || 1) } }, sub { 1 }, 1_000; #<==== Here there is a 3rd arg
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The output shown on Rosetta is correct

My TWO questions are:

1) Why can they pass in two arguments sometimes (two function refs) and three arguments other times (two function refs, and an integer count?

2) What value does the expression $_[2] // 100 assign to $n ?