Please fill in the missing blank for me please.

input	output
&	&
&lab;	&lab;
&lab;	<
???	&lab;

(aXML is format agnostic, so I really mean the output of aXML, and not how it would look if interpreted as HTML by a browser.)

It sounds like you changed aXML from

input	output
&	&
<post_include>path/to/lt</post_include>	<
&lab;	&lab;

to

input	output
&	&
&lab;	<
<post_include>path/to/lab</post_include>	&lab;

which means

sub plugin_name {
   my $text_to_return = ...;
   $text_to_return =~ s{<}{<post_include>path/to/lt</post_include>}g;
   return $text_to_return;
}
[download]

is now

my %subst = (
   '<'     => '&lab;',
   '&lab;' => '<post_include>path/to/lab</post_include>',
);
my ($subst_re) = map qr/$_/, join '|', map quotemeta, keys %subst;

sub plugin_name {
   my $text_to_return = ...;
   $text_to_return =~ s/($subst_re)/$subst{$1}/g;
   return $text_to_return;
}
[download]

You made things worse!

No matter whether it's based on regex, yacc or something, building the parser is the easy part — there are books that teach how to do that — it's building the language that's hard.