The problem is that I want to _reproduce_ under Perl the same order relation that the system level sort uses
What problem? You should have checked what order the system's sort uses.
$ cat >data
aaa2000@yahoo.com
aaa_2000@yahoo.com
aaa2000
aaa_2000
$ export LC_COLLATE=en_US.UTF-8
$ sort data
aaa2000
aaa_2000
aaa_2000@yahoo.com
aaa2000@yahoo.com
$ perl -le'use locale; chomp(@a=<>); print for sort @a;' data
aaa2000
aaa_2000
aaa_2000@yahoo.com
aaa2000@yahoo.com
$ export LC_COLLATE=C
$ sort data
aaa2000
aaa2000@yahoo.com
aaa_2000
aaa_2000@yahoo.com
$ perl -le'use locale; chomp(@a=<>); print for sort @a;' data
aaa2000
aaa2000@yahoo.com
aaa_2000
aaa_2000@yahoo.com
Whether the order makes sense or not, it's doing exactly what you want.