I'm sorry, but I couldn't resist improving (?) on this...

#!/usr/bin/perl -w
use strict;
use Getopt::Long;

my ($animalfile, $animal_regex) = ('animals.txt','');
GetOptions('animalfile=s' => \$animalfile);

(-f $animalfile) or die "Whoops!  File \"$animalfile\" doesn't exist.\
+n";
print "Using animal file: $animalfile\n";
open ANIMALS, $animalfile;
$animal_regex = "\\b(?:" . (join '|', grep { $_ } map { chomp; quoteme
+ta; } <ANIMALS>) . ")s?\\b";
close ANIMALS;

print <<PRINT;
Type in a sentence, and I'll translate it dontcha know.
Type quit when you're done, if you feel like it that is.
PRINT
while (chomp ($_ = <STDIN>))
{
    last if ($_ eq 'quit');
    s/$animal_regex/cow/iog;
    s/\b(?:yes|sure|okay)\b/you bet/ig;
    s/\.$/ dontcha know./;
    print "$_\n";
}
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