: marks the left side of it as an invocant to call a method on, when a method of an object is used. It is easier to understand how it works in the following example.
class C {
method meth($x) {
say "meth($x)";
}
}
my $o = C.new;
meth($o: 42); # The meth method of the $o object is called,
# it prints “meth(42)”
The form meth($o: 42) is equivalent to the classical form $o.meth(42). Note that you cannot omit a space following the colon (otherwise, it will be interpreted as a named argument).
Another common Perl 6 idiom for the use of : is to prevent having parentheses with method calls. The following two lines of code are equivalent:
say "abcd".substr: 1, 2; # bc say "abcd".substr(1, 2); # bc