From type/IO/Handle

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method out-buffer(--> Int:D) is rw

Controls output buffering and can be set via an argument to open. Takes an int as the size of the buffer to use (zero is acceptable). Can take a Bool: True means to use default, implementation-defined buffer size; False means to disable buffering (equivalent to using 0 as buffer size).

Lastly, can take a Nil to enable TTY-based buffering control: if the handle is a TTY, the buffering is disabled, otherwise, default, implementation-defined buffer size is used.

See flush to write out data currently in the buffer. Changing buffer size flushes the filehandle.

given 'foo'.IO.open: :w, :1000out-buffer {
    .say: 'Hello world!'; # buffered 
    .out-buffer = 42;       # buffer resized; previous print flushed 
    .say: 'And goodbye';
    .close; # closing the handle flushes the buffer 
}

From type/IO/CatHandle

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multi method out-buffer(|)

The IO::CatHandle type overrides this method to throw a X::NYI exception. If you have a good idea for how this method should behave, tell Rakudo developers about it!