.join can be argless, cleaning

src/core/Failure.pm

@@ -56,7 +56,7 @@ multi sub fail($payload) is hidden_from_backtrace {
     $fail
 }
 multi sub fail(*@msg) is hidden_from_backtrace {
-    my $payload = @msg == 1 ?? @msg[0] !! @msg.join('');
+    my $payload = @msg == 1 ?? @msg[0] !! @msg.join;
     die $payload if $*FATAL;
     my $fail := Failure.new(X::AdHoc.new(:$payload));
     my Mu $return := nqp::getlexcaller('RETURN');

src/core/Real.pm

@@ -73,7 +73,7 @@ my role Real does Numeric {
             push @frac_digits, @conversion[$frac.Int];
             $frac = $frac - $frac.Int;
         }
-        my Str $r = $int_part.base($base) ~ '.' ~ @frac_digits.join('');
+        my Str $r = $int_part.base($base) ~ '.' ~ @frac_digits.join;
         # if $int_part is 0, $int_part.base doesn't see the sign of self
         $int_part == 0 && self < 0 ?? '-' ~ $r !! $r;
     }

src/core/Str.pm

@@ -802,7 +802,7 @@ my class Str does Stringy { # declared in BOOTSTRAP
                       !! $p ~~ /<.lower>/  ?? $s.lc
                       !! $s;
         }
-        @chars.join('');
+        @chars.join;
     }
 
     method samespace(Str:D: Str:D $pat) {
@@ -1221,7 +1221,7 @@ sub unbase_bracket($base, @a) {
 }
 
 sub chrs(*@c) returns Str:D {
-    @c.map({.chr}).join('');
+    @c.map({.chr}).join;
 }
 
 sub substr-rw($s is rw, $from = 0, $chars = $s.chars - $from) {

src/core/control.pm

@@ -151,7 +151,7 @@ multi sub die(*@msg) is hidden_from_backtrace {
 
 multi sub warn(*@msg) is hidden_from_backtrace {
     my $ex := nqp::newexception();
-    nqp::setmessage($ex, nqp::unbox_s(@msg.join('')));
+    nqp::setmessage($ex, nqp::unbox_s(@msg.join));
     nqp::setextype($ex, nqp::const::CONTROL_WARN);
 #?if parrot
     nqp::bindattr($ex, Exception, 'severity', nqp::p6box_i(pir::const::EXCEPT_WARNING));