See the current Perl documentation for lib:locale.
Here is our local, out-dated (pre-5.6) version:

locale - Perl pragma to use and avoid POSIX locales for built-in operations

@x = sort @y; # ASCII sorting order
{
use locale;
@x = sort @y; # Locale-defined sorting order
}
@x = sort @y; # ASCII sorting order again

This pragma tells the compiler to enable (or disable) the use of POSIX locales for built-in operations (LC_CTYPE for regular expressions, and LC_COLLATE for string comparison). Each ``use locale'' or ``no locale'' affects statements to the end of the enclosing BLOCK.