Another way:

use strict;
use warnings;
use 5.010;

my @strings = qw{
    aB
    namesCanBeDifferentAny
    helloWorld
};


for my $str (@strings) {

    $str =~ s{
                (            
                    [a-z]*
                )
                (   
                    [A-Z]
                    [a-z]*
                )
             }
             {\U$1_$2}xmsg;
    say $str;
}

--output:--

A_B
NAMES_CAN_BE_DIFFERENT_ANY
HELLO_WORLD
[download]

...but alas it is very slow, producing half as many loops as happy.barney's lookaround. Who would have thought that a zero width match could be replaced by something?