A slight correction: the above code is going to print animal=pet. If you want to not use CGI.pm (bad, evil, you've been warned, etc.), then $pet is going to contain:
variable1=value1&variable2=value2...
Some fairly rudimentary parsing can get you what you need. However, then you might need to worry about how HTML encodes special characters (what if the user enters "<lephant" or something else silly?) which isn't too bad but you're quickly seeing why CGI.pm is probably a better choice.
Cheers,
Ardenstone

Update:
Here is some ugly not use strict-able code (it creates variables on the fly) that can parse out the environment string:

$myTmp = $ENV{'QUERY_STRING'};
@words = split '&',$myTmp;
my $counter = 0;

while ($words[$counter]) {
    my @tmpWords = split '=',$words[$counter];
    $tmpWords[1] =~ tr/+/ /;
    $tmpWords[1] =~ s/%([a-fA-F0-9][a-fA-F0-9])/pack("C", hex($1))/eg;
    my $tmpName = $tmpWords[0];
    $$tmpName = $tmpWords[1];
    $counter++;
}
[download]

Now that's ugly. But it should work.