The first part was stolen from
72251ce78c061 on the maint-5.18 branch.
C</$qr/p> was broken in Perl 5.18.0; the C</p> flag was ignored. This has
been fixed [perl #118213].
+=item *
+
+Starting in v5.18.0, a construct like C</[#](?{})/x> would have its C<#>
+incorrectly interpreted as a comment. The code block would be skipped,
+unparsed. This has been corrected.
+
+=item *
+
+Starting in Perl 5.001, a regular expression like C</[#$a]/x> or
+C</[#]$a/x> would have its C<#> incorrectly interpreted as a comment, so
+the variable would not interpolate. This has been corrected [perl #45667].
+
=back
=head1 Known Problems