sub isprime($) {
    my $num = $_[1];
    $primer = 1;
    $ValidChars = "2357";
    $str = $num;
    if (index($ValidChars, ord(substr $str, 0, 1)) == 1) {
        $primer = 0;
    }
    return $primer;
}
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That returns 1 unless the first character of the first argument (assuming you fix the already noticed error regarding $_[1]) equals 3. I think you are calling isprime only with single digit numbers, and want to return if the digit is a prime number. I'd write that as:

sub isprime {$_[0] =~ /^[2357]$/}
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although I would fix the name. Or just inline the test -- that's a lot clearer than any name I can think of.

if (($n != 1) && ($n != 0)) {
   if (isprime($n)) {
      $prime += $n;
   }
   if (!isprime($n)) {
      $comp += $n;
   }
}
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Too many ifs, and too many calls to isprime. I'd write that as:

if ($n =~ /^[2357]$/) {
    $prime += $n;
}
elsif ($n !~ /^[01]$/) {
    $comp += $n;
}
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Or, when I'm in the mood for something funky, as a ternary returning an lvalue.

$text.length
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That should be spelled length($text).