Forgive me, but the working examples do not seem to cover the use of \1 (etc) in the replacement expression.

It's covered in perlop

Unlike sed, we use the \<digit> form in only the left hand side. Anywhere else it's $<digit>.

Perl warns about it, too:

$ perl -wE '$_ = "foo"; s/(foo)/bar \1/; say'
\1 better written as $1 at -e line 1.
bar foo
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We're told that backreference patterns can be used in the substitution by specifying \1, \2 (etc), but it doesn't seem to work.

Who told you that?