Essentially, there are three buffering modes to be aware of:

• unbuffered
• line-buffered
• block-buffered

"line-buffered" is typically used with interactive things such as an interactive shell / terminal that your command line runs in.  "block-buffered" is the default, used with almost everything else. And "unbuffered" is only used for special purposes, if you explicitly request that the performance optimisation not be used.  Line-buffered output is flushed when encountering a newline in the stream, while block-buffered output is flushed when the block has filled up.

Put differently, if you for example redirect the output (an operation which is considered non-interactive) of your snippet which does output the "\n" to a file

$ 688477.pl >suffbuf
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and then try "tail -f suffbuf" (from another terminal) in an attempt to watch the lines being output, it wouldn't work either... (while it would with $|=1;).  I.e., before you get any output, you'd have to wait quite a while until the buffer has been filled up (typically several kByte), at which moment the whole buffer would be output at once as one block...