$ 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