Create a temporary file with Tempfile and upload from that
require 'net/ftp'
require 'tempfile'
tempfile = Tempfile.new('test')
my_data = "This is some text data I want to upload via FTP."
tempfile.puts my_data
ftp = Net::FTP.new('ftp.domain.com')
ftp.passive = true
ftp.login
ftp.chdir('/your/folder/name/here')
ftp.puttextfile(tempfile.path, 'my_data')
ftp.close
tempfile.close