sub mysplit {
    
    my $string = shift;
    my @parts;
    my $part = '';
    my $escaped = 0;
    
    for ($string =~ m/(.)/gs) {

        if ($_ eq '@') {
            unless ($escaped) {
                push @parts, $part;
                $part = '';
                next;
            }
        }
        if ($_ eq "#") {
            $escaped ^= 1;  # toggle state
        } else {
            $escaped = 0;   # reset
        }
        $part .= $_;
    }
    push @parts, $part;

    return @parts;
}

my @tests = (
    'one@two@three',
    '## is a hash and #@ is an arobace',
    '#@##@###@####@#####@',
);

for my $s (@tests) {
    print join(', ', mysplit($s) ), "\n";
}