Instead of splitting the two strings, why not use substr?

for my $i (0 .. length($str1)) {
    if (substr($str1, $i, 1) ne substr($str2, $i, 1)) {
        $val++;
    }
}
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That split is bound to be very expensive, both in terms of time and memory.

You could optimize that further, by looking at five letters at a time:

my $step = 5;
my $bound = int (length($str1) / $step);

for my $i (0 .. $bound) {
    if (substr($str1, ($i * $step), $step) ne (substr($str2, ($i * $st
+ep), $step)) {
        $val++;
    }
}
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Update: If you need to update $val for each character that's different, do something like this:

for my $i (0 .. $bound) {
    my $substr1 = substr($str1, ($i * $step), $step);
    my $substr2 = substr($str2, ($i * $step), $step);
    if ($substr1 ne $substr2) {
        for my $j (0 .. ($step - 1)) {
            if (substr($substr1, $j, 1) ne
                substr($substr2, $j, 1)) {
                $val++;
            }
        }
    }
}
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Uglier, yes, but still faster than split.