use strict;

my $header = shift @ARGV;
my $add = shift @ARGV;

$add =~ tr/+/ /;
$add =~ s/\%([A-Z0-9][A-Z0-9])/pack("C",hex($1))/eg;

$add .= ' ' unless $add =~ /\s$/;

# read headers
while (<>) {
    chomp;
    last if $_ eq '';    # end of headers!

    my ($h, $v) = /^(\S+): (.*)/;
    if (defined $h) {
        if ($h eq $header) {
            $v = $add . $v;
        }
        print "$h: $v\n";
    } else {
        print "$_\n"; ## weird
    }
}

# sep
print "\n";

# body
while (<>) {
    print;
}