In general \Q...\E is used in double-quote context (such as one gets inside m// and the first leg of s///) to quote characters (such as * and +) that would otherwise have special meaning in a regular expression. E.g.:

'aaaa+' =~ /(a+)/;     print "$1\n";
'aaaa+' =~ /(\Qa+\E)/; print "$1\n";
__END__
aaaa
a+
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The first regexp matches the string of a's, whereas the second one matches the substring 'a+'.