Well, see the code below from perldoc CGI. Isn't it obvious how to set the charset?

print $query->header(-type=>'image/gif', # <-think text/html
                     -nph=>1,
                     -status=>'402 Payment required',
                     -expires=>'+3d',
                     -cookie=>$cookie,
                     -charset=>'utf-7',  # <-charset
                     -attachment=>'foo.gif',
                     -Cost=>'$2.00');
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