Original code:

read(STDIN,$form_data,$ENV{'CONTENT_LENGTH'});
open (FILE,""something.log");
print $form_data;
close(FILE);
[download]

First, you are not doing anything with the file. Second, if you want to send html back you need to print "Content-type: text/html\n\n"

# Get form information
if ($ENV{'REQUEST_METHOD'} eq "GET") { # GET Method
    $form_data = $ENV{'QUERY_STRING'};
}
elsif ( $ENV{'CONTENT_LENGTH'} > 0 ) { # POST Methond
        read(STDIN, $form_data, $ENV{'CONTENT_LENGTH'});
}
else { exit; } # Error handling

# To print the form info to a file
open (FILE,">>something.log"); # Open the file in append mode
flock (FILE,2); # Exclusive lock 
print FILE $form_data; # Print the form to the file

# To print the file as a responce
open (FILE,"<something.log"); # Open the file in read mode
flock (FILE,1); # Sharded lock
print "Content-type: text/html\n\n"; # Print HTTP header
print <FILE>; # Print file

close(FILE); # Close and unlock the file
[download]

Hope that helps.

PS Not everyone can use CGI.pm. Some places do not like extra moduals to deal with, so it is always helpful to know the raw way of doing something.