From language/operators

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multi sub infix:<=:=>(Mu \a, Mu \b)

Container identity operator. Returns True if both arguments are bound to the same container. If it returns True, it generally means that modifying one will also modify the other.

my ($a, $b) = (1, 3);
say $a =:= $b;      # OUTPUT: «False␤» 
$b = 2;
say $a;             # OUTPUT: «1␤» 
$b := $a;
say $a =:= $b;      # OUTPUT: «True␤» 
$a = 5;
say $b;             # OUTPUT: «5␤»

The single argument version, called as a routine, will always return True:

say infix:<=:=>(42);    # OUTPUT: «True␤» 
say infix:<=:=>(False); # OUTPUT: «True␤»