I believe the package is libxul-dev. The following patch, applied to Mozilla::DOM's Makefile.PL resolves the dependency and allows a compile and test to work successfully (at least for me on Debian SID).

    --- Makefile.PL 2007-06-06 22:40:06.000000000 +0200
    +++ /home/reenen/tmp/Makefile.PL        2008-05-07 14:41:16.204005
+648 +0200
    @@ -14,6 +14,7 @@

     # someone mentioned adding iceweasel (debian), but I don't
     # know how it's called
    +my $icepkg = 'xulrunner-xpcom';
     my $mozpkg = 'mozilla-xpcom';
     my $ffpkg  = 'firefox-xpcom';
     my $mffpkg = 'mozilla-firefox-xpcom';
    @@ -21,6 +22,7 @@
     our %build_reqs = (
         'perl-ExtUtils-Depends'   => '0.205',
         'perl-ExtUtils-PkgConfig' => '1.07',
    +    $icepkg                   => '1.7',
         $mozpkg                   => '1.7',
         $ffpkg                    => '1.0',
         $mffpkg                   => '1.0',
    @@ -40,7 +42,8 @@
         exit 1; # not reached
     }

    -my %pkgcfg = ExtUtils::PkgConfig->find("$mozpkg >= " . $build_req
+s{$mozpkg},
    +my %pkgcfg = ExtUtils::PkgConfig->find( "$icepkg >= " . $build_re
+qs{$icepkg},
    +                                       "$mozpkg >= " . $build_req
+s{$mozpkg},
                                            "$ffpkg >= " . $build_reqs
+{$ffpkg},
                                            "$mffpkg >= " . $build_req
+s{$mffpkg});
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