use HTTP::Request;

##</code><code>##

BEGIN {
    use HTTP::Request;
    $HTTP::Headers::TRANSLATE_UNDERSCORE = 0;
}

##</code><code>##

#!/usr/bin/env perl -l

use strict;
use warnings;

BEGIN {
    use HTTP::Request;
    $HTTP::Headers::TRANSLATE_UNDERSCORE = 0;
}

use Data::Dump;

my $r = HTTP::Request->new;

my $header = $r->header('__X_Y__', 1);

dd $r;

##</code><code>##

bless({
  _content => "",
  _headers => bless({ "--x-y--" => 1, "::std_case" => { "--x-y--" => "--X-Y--" } }, "HTTP::Headers"),
  _method  => undef,
  _uri     => undef,
}, "HTTP::Request")

##</code><code>##

bless({
  _content => "",
  _headers => bless({ "::std_case" => { __x_y__ => "__X_Y__" }, "__x_y__" => 1 }, "HTTP::Headers"),
  _method  => undef,
  _uri     => undef,
}, "HTTP::Request")

##</code><code>##

...
# The $TRANSLATE_UNDERSCORE variable controls whether '_' can be used
# as a replacement for '-' in header field names.
our $TRANSLATE_UNDERSCORE = 1 unless defined $TRANSLATE_UNDERSCORE;
...
$field =~ tr/_/-/ if $TRANSLATE_UNDERSCORE;
...