mac_addr = '50:E5:49:EC:7B:A4'
puts "
DhcpfixedIpAddress[1] = {}
DhcpfixedIpAddress[1]["MacAddr"] = "mac_addr"
"

out - 
sorc@ubuntu-minimal:~/dsr500$ ruby write_clients.rb
write_clients.rb:4: syntax error, unexpected tCONSTANT, expecting $end
DhcpfixedIpAddress[1]["MacAddr"] = "mac_addr"
                              ^