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unless
Combined from primary sources listed below.

In Control flow§

See primary documentation in context for unless

When you get sick of typing "if not (X)" you may use unless to invert the sense of a conditional statement. You cannot use else or elsif with unless because that ends up getting confusing. Other than those two differences unless works the same as if:

unless 1 { "1 is false".say }  ; # does not say anything, since 1 is true 
unless 1   "1 is false".say    ; # syntax error, missing block 
unless 0 { "0 is false".say }  ; # says "0 is false" 
unless 42.say and 1 { 43.say } ; # says "42" but does not say "43" 
43.say unless 42.say and 0;      # says "42" and then says "43" 
43.say unless 42.say and 1;      # says "42" but does not say "43" 
 
$_ = 1; unless 0 { $_.say }           ; # says "1" 
$_ = 1; unless 0 -> $_ { $_.say }     ; # says "0" 
$_ = 1; unless False -> $a { $a.say } ; # says "False" 
 
my $c = 0; say (1, (unless 0 { $c += 42; 2; }), 3, $c); # says "(1 2 3 42)" 
my $d = 0; say (1, (unless 1 { $d += 42; 2; }), 3, $d); # says "(1 3 0)"