G'day morgon,

"... how can I turn it off so I can send headers with leading double-underscores?"

The short answer first. Somewhere near the start of your code you'll have:

use HTTP::Request;
[download]

Change that line to:

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

That's the fix. Read on for details ...

My test:

#!/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;
[download]

With just "use HTTP::Request;", instead of that BEGIN block, I get:

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

With the BEGIN block, as shown, I get:

bless({
  _content => "",
  _headers => bless({ "::std_case" => { __x_y__ => "__X_Y__" }, "__x_y
+__" => 1 }, "HTTP::Headers"),
  _method  => undef,
  _uri     => undef,
}, "HTTP::Request")
[download]

"Why is that ..."

A little investigation, using the published documentation, provided the answer.

In HTTP::Request, under "$r->header( $field => $value )", you'll see:

"This is used to get/set header values and it is inherited from HTTP::Headers ..."

In HTTP::Headers, under "$h->header( $field => $value )", you'll see:

"To make the life easier for perl users who wants to avoid quoting before the => operator, you can use '_' as a replacement for '-' in header names."

You'll also see similar comments for other methods.

Following the Source link (at the top of the page), shows:

...
# 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;
...
[download]

I searched the documentation of both HTTP::Request and HTTP::Headers for "TRANSLATE_UNDERSCORE": no matches were found. Perhaps you'd care to raise a bug report for a documentation enhancement.

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