I too found the phrasing confusing but it is correct as stated

A value of form_data (an error) is treated as "application/x-www-form-urlencoded"

A value of form-data is treated as "multipart/form-data"

#!/usr/bin/perl --
use strict;
use warnings;
use LWP;

my $ua = LWP::UserAgent->new;
$ua->add_handler("request_send",  sub { shift->dump; return });
$ua->add_handler("response_done", sub { shift->dump; return });
$ua->timeout( 0.1 );

my $res = $ua->post('http://localhost:8012/',
#~     Content_Type => 'form_data', ## treated as "application/x-www-f
+orm-urlencoded"
    Content_Type => 'form-data',  ## treated as "multipart/form-data"
#~     Content_Type => 'multipart/form-data', 
    Content => [
        Filedata => [
            __FILE__,
            'Hello.txt',
            Content_Type => 'text/plain',
        ],
        submit => 'Submit',
    ],
);
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