But it is not difficult to translate the generated C code to Gtk2.pm. The APIs are similar.

The APIs are identical only the calling convention is different, but there is also no point in doing the translation yourself, it misses the point of using Glade, the idea behind Glade is for it to generate code for you
Glade::PerlGenerate
glade2perl - Builds Gtk-Perl sources from a Glade file
glade2perl-2 - Builds Gtk2-Perl sources from a Glade-2 file
Glade2::Widget - Creates a new widget from a .glade file. It accepts the following parameters.
Gtk2::GladeXML - Create user interfaces directly from Glade XML files.
Gtk2::GladeXML::Simple - A clean object-oriented interface to Gtk2::GladeXML