Why would HMAC SHA-1 return a different digest with the same input?

#!/usr/bin/perl -w

use MIME::Base64 qw(encode_base64);
use Digest::HMAC_SHA1;
use String::Escape qw( printable unprintable );

sub _ascii_to_hex {
    (my $str = shift) =~ s/(.|n)/sprintf("%02lx", ord $1)/eg;
    return $str;
}

sub _encode {
    my ( $aws_secret_access_key, $str ) = @_;
    print "secret key hex: "._ascii_to_hex($aws_secret_access_key)."n";
    my $hmac = Digest::HMAC_SHA1->new($aws_secret_access_key);
    $hmac->add($str);
    my $digest = $hmac->digest;
    print "cleartext hex: "._ascii_to_hex($str)."n";
    print "digest hex: "._ascii_to_hex($digest)."n";
    my $b64 = encode_base64( $digest, '' );
    print "encoded: ".$b64."n";
}

my $secret = "abcd1234";
my $cleartext = "GETnnnFri, 12 Dec 2008 10:08:51 GMT+00:00n/";
_encode($secret, $cleartext);

$ ./testhmac.pl
secret key hex: 6162636431323334
cleartext hex: 4745540a0a0a4672692c2031322044656320323030382031303a30383a353120474d542b30303a30300a2f
digest hex: 63308f9b8a198440d6d8685a3f3f70d0aab02f68
encoded: YzCPm4oZhEDW2GhaPz9w0KqwL2g=

$ s3ls --access-key=foobar --secret-key=abcd1234
...
secret key hex: 6162636431323334
cleartext hex: 4745540a0a0a4672692c2031322044656320323030382031303a30383a353120474d542b30303a30300a2f
digest hex: c0da50050c451847de7ed055c5286de584527a22
encoded: wNpQBQxFGEfeftBVxSht5YRSeiI=

use Digest::SHA qw(hmac_sha1_hex);
my $hash = hmac_sha1_hex($data, $key);

secret key hex: abcd...1234

_ascii_to_hex("blahblahblah")