They work at a very low level. In the tokeniser, I believe. That would explain why they fail if you do anything odd with them, such as follow a \Q with a \U 
>perl -e"print qq{\Q\U,a}"
\,A

[download]

(\Q doubled \U's slash without realising it was part of an escape.)

In my opinion, they are primarily meant to be used when followed by a variable.

Update: Oops, it's actually working right. The "\" is for the comma. I wasn't awake yet :(

In reply to Re: Interactions of \Q, \U, \L and \E by ikegami
in thread Interactions of \Q, \U, \L and \E by ig

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