End of Life Ruby 3.1 has reached end of life and gets no more fixes. Switch to Ruby 4.0 →

class method

ENV.dup

ENV.dup # raises TypeError

static VALUE
env_dup(VALUE obj)
{
    rb_raise(rb_eTypeError, "Cannot dup ENV, use ENV.to_h to get a copy of ENV as a hash");
}

Raises TypeError, because ENV is a singleton object. Use to_h to get a copy of ENV data as a hash.

See also .delete_if .each .delete .each_key also in 4.1 · 4.0 · 3.4 · 3.3 · 3.2