"Can't spawn "telnet x.x.x.x"... But the code works well under linux..."

then

",it also works well under ActivePerl Any help?"

Perhaps, for the sake of clarification you could explain which platform/Perl distribution cases this error? Do you have a telnet executable in your path on the system where this fails? See system regarding passing arguments. Also, consider reading How do I post a question effectively?

Note that telnet isn't secure, though if you insist on using it try using the Net::Telnet module to ensure portability.