If I understood clearly you cannot use module CGI. If it is, there is a one way to get form data and store them:

if($ENV{'REQUEST_METHOD'} eq 'GET') {   $query=$ENV{'QUERY_STRING'} }
elsif($ENV{'REQUEST_METHOD'} eq 'POST') { sysread(STDIN,$query,$ENV{'C
+ONTENT_LENGTH'}) }
else { die 'Unsupported method: $ENV{'REQUEST_METHOD'}' }

# Now $query consists a string with pairs name=value which 
# are sticked '&'

$query =~ s/&/\n/g;

open(FILE, '/your/file') or die 'Cannot open file: $!';
print FILE $query;
close FILE;
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SV* sv_bless(SV* sv, HV* stash);