It may be helpful to consider  \Q...\E quotemeta interpolation in a string-only context, entirely divorced from regexification. These escapes control meta-quoting in any double-quote evaluation, of which a regex is only one instance.

c:\@Work\Perl\monks>perl -wMstrict -le
"my $interp = 'a.b';
 my $string = qq{--\Q$interp\E--};
 print qq{'$string'};
 ;;
 my $strung = qq{--$interp--};
 print qq{'$strung'};
"
'--a\.b--'
'--a.b--'
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