also flip-flop can be used (look for the scalar context usage):

use strict;
use warnings;

while (<DATA>){
    print  /^cat/ .. /^\^cat/ ? "#$_" : $_;
}

__DATA__
mouse
cat
1
2
3
^cat 
deer
eel
cat
furbaby
elk
^cat
dog
fish
[download]

L*

PS ^\^ is actually a cat :)

PPS overlooked previous answer was already using flip-flop.. let rephrase: you can also use flip-flop directly :)