When continued_fraction($x,$y,$z) is called, the parameters $x, $y, $z are
assigned to the list @_ which is visible locally inside the sub and
can be addressed in the usual way like $_, $_...
Here, @_[0,1] is an array-slice equivalent to the list ($_, $_).
BTW: Accessing @_ is "call-by-reference" style. Assigning to i.e $_ inside the sub would change callers $x in the example above.
Usually, you would create a local copy, i.e. my (@args) = @_;. Accessing @_ might be used here for speed-optimisation or because of bad style?
$n = $_ // 100;
Perl historically had no strict function/sub signatures, so you can pass any number of arguments
without Perl complaining - that feature
was added recently (Perl 5.20, experimental).
is the same as
$n = defined($_) ? $_ : 100;
So, if the third - obviously optional parameter is given, it is used. If not, the default value 100 is assigned to $n