use HTTP::Daemon;
use LWP::UserAgent;
use Compress::Zlib;
use CGI::Carp qw(fatalsToBrowser);
my @ext = qw(x-gtar x-gzip x-tar zip image zip z gz tz taz tgz lzh arj arc tar cab wmz yfs wsz exe uu xxe bhx b64 hqx mim);
my $port = 8080;
$SIG{PIPE} = 'IGNORE';
my $server = HTTP::Daemon->new( LocalPort => $port ) or
die "Can't start server ($@)" unless defined $server;
my $ua = LWP::UserAgent->new;
$ua->default_header('My header' => "value") # вот тут я сломал голову
while (my $conn = $server->accept) {
    while (my $request = $conn->get_request) {
           $conn->autoflush;
                $request -> remove_header('Proxy-Connection');
        $request -> remove_header('Keep-Alive');
        $request -> header(Connection=>'close');
           my $resp = $ua->simple_request($request);
           my $c_cont = $resp->content;
        if ( $resp->content_encoding || (grep { $_ =~ /$resp->content_type/g } @ext) || length($c_cont) < 3000 ) {
        $conn->send_response($resp);
        }
        else {
             $c_cont_zip = Compress::Zlib::memGzip($c_cont);
             $resp->content($c_cont_zip);
             $resp->content_encoding('gzip');
             $conn->send_response($resp);
             }
     }
    print STDERR "Close: ", $conn->reason, "\n";
    $conn->close;
    undef $conn;
}

 while (my $conn = $server->accept) {
    my $ua = LWP::UserAgent->new;
    my $str = do {my $fh = open "<filename.txt"; local $/ = undef; <$fh>};
    chomp $str;
    $ua->default_header('My header' => $str)
    .................
}

my $str = do {open my $fh, "<filename.txt"; local $/ = undef; <$fh>};

use strict;
use warnings;