You can stack the necessary commands in the .perldb file to automate the turning on/turning off of Devel::Carnivore. Here's the program stripped down:

use strict;
use warnings;

my $h;
{
    my %h;                     # watch from here ...
    $h{fname} = "Foo";
    $holder = \%h;    
}

$holder->{lname} = 'Bar';       

$holder->{email} = 'foo@bar.com';   # ... to before here
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And here's .perldb to watch %h as noted:

push @DB::typeahead, (split /\n/, <<EOS);
# Load Carnivore
use Devel::Carnivore
# Line to start watching on
c 7
watch(\\%h) 
# run to point where we want to switch off
c 13
# do so
unwatch \$h
# run to end of program
c
# quit debugger
q
EOS
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Run this as perl -d <your program>. The output:

01:26 PM $ perl5.8.5 -d z.pl

Loading DB routines from perl5db.pl version 1.27
Editor support available.

Enter h or `h h' for help, or `man perldebug' for more help.

main::(z.pl:4):    my $h;
main::(z.pl:5):    {
auto(-12)  DB<1> use Devel::Carnivore

auto(-11)  DB<2> # Line to start watching on

auto(-10)  DB<3> c 7
main::(z.pl:7):        $h{fname} = "Foo";
auto(-9)  DB<4> watch(\%h) 
# variable is now under observation

auto(-8)  DB<5> # run to point where we want to switch off

auto(-7)  DB<6> c 13
> "fname" changed from "" to "Foo"  at z.pl line 7
> "lname" changed from "" to "Bar"  at z.pl line 11
main::(z.pl:13):    $h->{email} = 'foo@bar.com';   # ... to before her
+e
auto(-6)  DB<7> # do so

auto(-5)  DB<8> unwatch $h
# mission completed

auto(-4)  DB<9> # run to end of program

auto(-3)  DB<10> c
Debugged program terminated.  Use q to quit or R to restart,
  use O inhibit_exit to avoid stopping after program termination,
  h q, h R or h O to get additional info.  
auto(-2)  DB<10> # quit debugger

auto(-1)  DB<11> q
01:26 PM $
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The problem, of course, is that you're dependent on line numbers, but there's an example of how to automate the debugger. Removing the comments will of course give you nicer output as well.