It's difficult to know what exactly you mean with such limited information, but I think that you probably want something along these lines:

use CGI qw(:cgi);
use strict;

my $flatfile = "somefile.txt";
my $query = CGI->new();
my $text = $query->param("text_box");

open(FLATFILE, ">>somefile.txt") or die "Open failed, $!";
print FLATFILE "$text\n\n";
close(FLATFILE);

print "Content-type: text/html\n\n";
print "Success!";
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This code assumes that A. you want to append the data to a growing file with each submission, and B. you only want the data from a single form field called "text_box".

Update: Oh, and it also assumes that you want 2 line breaks between each entry in the text file.

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