Greatest common divisor

Iterative

sub gcd (Int $a is copy, Int $b is copy) {
   $a & $b == 0 and fail;
   ($a, $b) = ($b, $a % $b) while $b;
   return abs $a;
}

Recursive

multi gcd (0,      0)      { fail }
multi gcd (Int $a, 0)      { abs $a }
multi gcd (Int $a, Int $b) { gcd $b, $a % $b }

Concise

my &gcd = { ($^a.abs, $^b.abs, * % * ... 0)[*-2] }

Actually, it's a built-in infix

my $gcd = $a gcd $b;

Because it's an infix, you can use it with various meta-operators:

[gcd] @list;         # reduce with gcd
@alist Zgcd @blist;  # lazy zip with gcd
@alist Xgcd @blist;  # lazy cross with gcd
@alist »gcd« @blist; # parallel gcd