I clicked Submit to early. Here is the actual patch.
What's more, I think this patch has a surprise : If types.db is cached in-memory, then $DB_FILE is change, the new $DB_FILE won't be loaded.
diff -rubN MIME-Types-2.09-orig/lib/MIME/Types.pm MIME-Types-2.09-PG/lib/MIME/Types.pm
--- MIME-Types-2.09-orig/lib/MIME/Types.pm 2014-09-14 12:12:24.000000000 -0400
+++ MIME-Types-2.09-PG/lib/MIME/Types.pm 2015-06-11 14:18:33.000000000 -0400
@@ -19,6 +19,8 @@
my %typedb;
sub new(@) { (bless {}, shift)->init( {@_} ) }
+our $DB_FILE;
+
sub init($)
{ my ($self, $args) = @_;
keys %typedb or $self->_read_db($args);
@@ -32,6 +34,7 @@
my $only_iana = $args->{only_iana};
my $db = $args->{db_file}
+ || $DB_FILE
|| File::Spec->catfile(dirname(__FILE__), 'types.db');
local *DB;
diff -rubN MIME-Types-2.09-orig/lib/MIME/Types.pod MIME-Types-2.09-PG/lib/MIME/Types.pod
--- MIME-Types-2.09-orig/lib/MIME/Types.pod 2014-09-14 12:12:24.000000000 -0400
+++ MIME-Types-2.09-PG/lib/MIME/Types.pod 2015-06-11 14:40:50.000000000 -0400
@@ -244,6 +244,23 @@
=back
+=head1 VARIABLES
+
+=over 4
+
+=item B<$MIME::Types::DB_FILE>
+
+Location of C<types.db>, the database of known extensions. Normally,
+MIME::Types looks in the directory were C<Types.pm> is installed. You may
+use this variable to specify another location for C<types.db>.
+
+example:
+
+ $MIME::Types::DB_FILE = File::Spec->catfile( $datadir, "types.db" );
+
+=back
+
+
=head1 SEE ALSO
This module is part of MIME-Types distribution version 2.09,
diff -rubN MIME-Types-2.09-orig/t/20types.t MIME-Types-2.09-PG/t/20types.t
--- MIME-Types-2.09-orig/t/20types.t 2014-09-14 12:12:24.000000000 -0400
+++ MIME-Types-2.09-PG/t/20types.t 2015-06-11 14:34:26.000000000 -0400
@@ -98,3 +98,5 @@
ok($r7, 'type sheet');
ok($r7->isBinary);
ok(!$r7->isText);
+
+
diff -rubN MIME-Types-2.09-orig/t/23db_file.t MIME-Types-2.09-PG/t/23db_file.t
--- MIME-Types-2.09-orig/t/23db_file.t 1969-12-31 19:00:00.000000000 -0500
+++ MIME-Types-2.09-PG/t/23db_file.t 2015-06-11 14:26:46.000000000 -0400
@@ -0,0 +1,22 @@
+#!/usr/bin/env perl
+#
+# Test reporting warnings, errors and family.
+#
+
+use strict;
+use warnings;
+
+use lib qw(lib t);
+
+use Test::More tests => 3;
+
+use FindBin;
+use File::Spec;
+use MIME::Types;
+
+local $MIME::Types::DB_FILE = File::Spec->catfile( $FindBin::Bin, 'fake-types.db' );
+my $a = MIME::Types->new;
+isa_ok( $a, 'MIME::Types' );
+
+is($a->mimeTypeOf('my.honk')->type, 'application/x-honk');
+is($a->mimeTypeOf('my.pdf')->type, 'application/x-not-pdf');
diff -rubN MIME-Types-2.09-orig/t/fake-types.db MIME-Types-2.09-PG/t/fake-types.db
--- MIME-Types-2.09-orig/t/fake-types.db 1969-12-31 19:00:00.000000000 -0500
+++ MIME-Types-2.09-PG/t/fake-types.db 2015-06-11 14:32:51.000000000 -0400
@@ -0,0 +1,8 @@
+2:EXTENSIONS
+honk;application/x-honk
+pdf;application/x-not-pdf
+
+2:application::
+x-honk;honk;
+x-not-pdf;pdf;
+
RT for rt.cpan.org