Here is a list of all documented variables with links to the struct/union documentation for each field:

warn_mismatched_indentation : pm_parser
warning_list : pm_parser
wbuf : rb_io
wrap_struct_name : rb_data_type_struct
write : pm_shareable_constant_node
write_lock : rb_io
write_name : pm_call_and_write_node, pm_call_operator_write_node, pm_call_or_write_node
writeconv : rb_io
writeconv_asciicompat : rb_io
writeconv_initialized : rb_io
writeconv_pre_ecflags : rb_io
writeconv_pre_ecopts : rb_io