I updated the code to read the file content using "File::Slurp".

#!/usr/bin/perl

use strict;
use warnings;
use LWP::UserAgent;
use File::Slurp;
use Data::Dumper;

my $ua = LWP::UserAgent->new();

my $server_endpoint = "https://www.tothesite.com/place/44498/";
 
# set custom HTTP request header fields
my $req = HTTP::Request->new(POST => $server_endpoint);

$req->headers();
print Dumper( $req->headers() );

$req->header('x-auth-token' => 'Token e45ttkfksj48sdfj4jd9abcdd');
 
# add POST data to HTTP request body
my $filename = "first_text.txt";
my $data_file = read_file( $filename, { binmode => ':raw' } );

$req->content($data_file);
 
my $resp = $ua->request($req);

if ($resp->is_success) {
    
    my $message = $resp->decoded_content;
    print "Received reply: $message\n";
    
}else {
    
    print "HTTP POST error code: ", $resp->code, "\n";
    print "HTTP POST error message: ", $resp->message, "\n";
    
}
[download]

Now I am getting this error:
HTTP POST error code: 500
Do you know if this line is code syntax correct?
 $req->header('x-auth-token' => 'Token e45ttkfksj48sdfj4jd9abcdd');