You can doi it like this:

use warnings;
use strict;

# Create a test file
open outFile, ">", "file1.txt";
print outFile <<end;
rabbit 1
hen    0
pig    2
cow    2
sheep  5
end
close outFile;

my %fed;
$fed{$1}++ while <DATA> =~ m/^(\w+)\s/;

local @ARGV = ('file1.txt');
local $^I = '.bak';
while (<>) {
  m/^(\w+)\s+(\d+)$/;
  printf "$1\t%d\n", exists $fed{$1} ? $2+1 : $2;
}

__DATA__
rabbit feed 10kg used on 21/10/2005 by consumer1
cow feed 100kg used on 11/11/2005 by consumer1
sheep feed 50kg used on 24/11/2005 by consumer3
[download]

file1.txt contains:

rabbit    2
hen    0
pig    2
cow    3
sheep    6
[download]