Почему добавление общего суффикса меняет порядок сопоставления в локали en_US?

Следующий код

#!/usr/bin/perl

use strict;
use warnings;

my $s1 = 'aaa2000@yahoo.com';
my $s2 = 'aaa_2000@yahoo.com';
my $s3 = 'aaa2000';
my $s4 = 'aaa_2000';

no locale;

print "\nNO Locale:\n\n";

if ($s1 gt $s2) {print "$s1 is > $s2\n";}
if ($s1 lt $s2) {print "$s1 is < $s2\n";}
if ($s1 eq $s2) {print "$s1 is = $s2\n";}

if ($s3 gt $s4) {print "$s3 is > $s4\n";}
if ($s3 lt $s4) {print "$s3 is < $s4\n";}
if ($s3 eq $s4) {print "$s3 is = $s4\n";}

use locale;

print "\nWith 'use locale;':\n\n";

if ($s1 gt $s2) {print "$s1 is > $s2\n";}
if ($s1 lt $s2) {print "$s1 is < $s2\n";}
if ($s1 eq $s2) {print "$s1 is = $s2\n";}

if ($s3 gt $s4) {print "$s3 is > $s4\n";}
if ($s3 lt $s4) {print "$s3 is < $s4\n";}
if ($s3 eq $s4) {print "$s3 is = $s4\n";}

выводит

NO Locale:

aaa2000@yahoo.com is < aaa_2000@yahoo.com
aaa2000 is < aaa_2000

With 'use locale;':

aaa2000@yahoo.com is > aaa_2000@yahoo.com
aaa2000 is < aaa_2000

, который я не могу понять: в то же время , при использовании locale, есть И a@yahoo.com > b@yahoo.com? !!

Locale is $ locale
LANG=en_US.UTF-8
LC_CTYPE="en_US.UTF-8"
LC_NUMERIC="en_US.UTF-8"
LC_TIME="en_US.UTF-8"
LC_COLLATE="en_US.UTF-8"
LC_MONETARY="en_US.UTF-8"
LC_MESSAGES="en_US.UTF-8"
LC_PAPER="en_US.UTF-8"
LC_NAME="en_US.UTF-8"
LC_ADDRESS="en_US.UTF-8"
LC_TELEPHONE="en_US.UTF-8"
LC_MEASUREMENT="en_US.UTF-8"
LC_IDENTIFICATION="en_US.UTF-8"
LC_ALL=