email form and smtp server

[makeformwork]

hello,
i am trying to get the following to work together to return email from a form. (it's kind of likely that a gap in my understanding is where the failure is happening!)

aprelium server
perl interpreter
bnbform.cgi (big nose bird html to email form)
QK SMTP Server 3
all on windows xp

my form calls bnbform.cgi ok (so I think i've got perl set up correctly) but I get an HTTP 500 message (and no email)

the cgi log shows the following message

"Can't modify constant item in scalar assignment at bnbform.cgi line 67, near ""localhost";"
Execution of bnbform.cgi aborted due to compilation errors.
CGI: [C:\Perl\bin\perl.exe bnbform.cgi ]
URI: /bnbform.cgi Broken pipe"

at line 67 i have set SMTP_SERVER="localhost"; (please let me know if pasting the rest of the code for bnbform would be helpful in answering my question)
and i was presuming that that was good enough to find my SMTP server?
maybe i've missed something about how to set up and run a local SMTP server?

cheers

[aprelium]