mouse
cat
1
2
3
^cat 
deer
eel
cat
furbaby
elk
^cat
dog
fish

##</code><code>##

mouse
#cat
#1
#2
#3
#^cat 
deer
eel
#cat
#furbaby
#elk
#^cat
dog
fish

##</code><code>##

 my $comment=0;
 for (@a) {
  $comment++ if /^cat\b/;
  $_ = '#'.$_ if $comment;
  $comment-- if /^\#\^cat\b/;
 }