\input expl3-generic
\ExplSyntaxOff  % warning on this line
\ExplSyntaxOn
\tl_new:N
  \g_example_tl
\tl_gset:Nn
  \g_example_tl
  { Hello,~ }
\ExplSyntaxOn  % warning on this line
\tl_gput_right:Nn
  \g_example_tl
  { world! }
\tl_use:N
  \g_example_tl