According to perlre:

\E        end case modification (think vi)
\Q        quote (disable) pattern metacharacters till \E
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Depending on what sort of text you want entered in $matchtext, this could be great or a disaster. Assuming that there were a few lines of code between the regex and the initialization,it would be pretty easy to forget that metacharacters are disabled. So if you take this route, make sure to leave a note for yourself where you set the variable. Running the wrong regex to diddle a bunch of files can ruin your day. (How good are your backups?)

TGI says moo