From language/operators

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multi sub prefix:<~>(Any --> Str:D)

String context operator.

Coerces the argument to Str by calling the Str method on it.

From language/operators

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multi sub infix:<~>(Any,   Any)
multi sub infix:<~>(Str:D, Str:D)
multi sub infix:<~>(Buf:D, Buf:D)
multi sub infix:<~>(Blob:D $a, Blob:D $b)
multi sub infix:<~>(Junction:D \a, Junction:D \b)

This is the string concatenation operator, which coerces both arguments to Str and concatenates them. If both arguments are Buf, a combined buffer is returned.

say 'ab' ~ 'c';     # OUTPUT: «abc␤» 
my $bob = Blob.new([1,2,3]);
my $bao = Blob.new([3,4,5]);
say $bao ~ $bob;     # OUTPUT: «Blob:0x<03 04 05 01 02 03>␤» 

The arity-1 version of this operator will be called when the hyper version of the operator is used on an array or list with a single element, or simply an element

say [~] Blob.new([3,4,5]);     # OUTPUT: «Blob:0x<03 04 05>␤» 
say [~] 1|2;                   # OUTPUT: «any(1, 2)␤» 

From type/Junction

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multi sub infix:<~>(Str:D $a, Junction:D $b)
multi sub infix:<~>(Junction:D $a, Str:D $b)
multi sub infix:<~>(Junction:D \a, Junction:D \b)

The infix ~ concatenation can be used to merge junctions into a single one or merge Junctions with strings. The resulting junction will have all elements merged as if they were joined into a nested loop:

my $odd  = 1|3|5;
my $even = 2|4|6;
 
my $merged = $odd ~ $even;
say $merged; # OUTPUT: «any(12, 14, 16, 32, 34, 36, 52, 54, 56)␤» 
 
say "Found 34!" if 34 == $merged; # OUTPUT: «Found 34!␤» 
my $prefixed = "0" ~ $odd;
say "Found 03" if "03" == $prefixed; # OUTPUT: «Found 03!␤» 
 
my $postfixed = $odd ~ "1";
say "Found 11" if 11 == $postfixed; # OUTPUT: «Found 11!␤» 

On the other hand, the versions of ~ that use a string as one argument will just concatenate the string to every member of the Junction, creating another Junction with the same number of elements.