Thanks for that. I have added the debug and got the output below.
Sadly it did not help a lot and even confused me
because I got the same messages about SMTP authentication when it was both 'on' and 'off'.
Perhaps the last message about Unexpected EOF on command channel
is the one I sohuld be thinking about.
Any thoughts on this will be welcome!

send_em no attachments
Net::SMTP>>> Net::SMTP(2.29)
Net::SMTP>>> Net::Cmd(2.26)
Net::SMTP>>> Exporter(5.58)
Net::SMTP>>> IO::Socket::INET(1.29)
Net::SMTP>>> IO::Socket(1.29)
Net::SMTP>>> IO::Handle(1.25)
Net::SMTP=GLOB(0x1d66690)<<< 220 smtp807.mail.ird.yahoo.com ESMTP
Net::SMTP=GLOB(0x1d66690)>>> EHLO localhost.localdomain
Net::SMTP=GLOB(0x1d66690)<<< 250-smtp807.mail.ird.yahoo.com
Net::SMTP=GLOB(0x1d66690)<<< 250-AUTH LOGIN PLAIN XYMCOOKIE
Net::SMTP=GLOB(0x1d66690)<<< 250-PIPELINING
Net::SMTP=GLOB(0x1d66690)<<< 250 8BITMIME
send_em smtp <Net::SMTP=GLOB(0x1d66690)>
Net::SMTP=GLOB(0x1d66690)>>> MAIL FROM:<correct e-mial address>
Net::SMTP=GLOB(0x1d66690)<<< 530 authentication required - "Your email could not be sent.</br To fix this you must make a simple change to your email (known as SMTP authentication).
For advice visit www.btyahoo.com/smtp"
send_em A en <0> <The operation completed successfully.
> result of smtp send ma <>
Net::SMTP=GLOB(0x1d66690)>>> RCPT TO:<correct e-mial address>
Net::SMTP: Unexpected EOF on command channel at email-jun09-b.pz line 159
send_em B en <6> <The handle is invalid.
> result of smtp send ma <> to <0>