Remove literal backslash from Perl \Q ... \E.
The behavior changed sometime after Perl 5.8.9, and "man perlre" says it
"may lead to confusing results." Per buildfarm member gaur. This
repairs commit
a7a7be1f2fa6b9f0f48e69f12256d8f588af729b.
Discussion: https://postgr.es/m/
20210629053627.GA2061079@rfd.leadboat.com