If you are using Perl, you really don't need to also use SSI, unless your ISP has a bunch of prebuit SSI's for you to use. Here's my untested code for this:

#!/usr/bin/perl -w
use strict;
use CGI qw(:standard);

print start_html();

open( TEXT, "</blah/header.txt" ) or
    die "missing text file: $!";
my @insertion;
while( <TEXT> )
{
    chomp;
    push( @insertion, $_ );
}
close TEXT;
print p( join( '', @insertion ) );
print end_html();
[download]

or something like that. There's lot of ways to do it, but CGI is a good habit to get into for all of them.