On second thought, the only reason you have a closure here at all is to preserve state between calls. Use a state variable then, and the more normal function definition syntax.

sub compress ($x) {
    state $previous;
    if $x ne $previous {
        return $x;
    }
    else {
        return;
    }
}
[download]

Update: I think the if is way too long, but can't remember if there's a context-safe way to return nothing in Perl 6 (that isn't "fail"), to golf out the "return" and say something like return $x ne $previous ?? $x !! $NOTHING_AT_ALL;. In the current application () is probably safe.

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