for( split //, $a ){
    $sum += $_;
}

my @array = split //, $a;
for( my $i = 0; $i <= $#array; $i++ ){ }

my @array = ('a'..'f');
for( my $i = 0; $i <= $#array; $i++ ){
    print "Iteration über Index: $i\n";
}

# vs.

for( @array ){
    print "Iteration über Element: $_\n";
}