It means that either $dna has no value assigned to it, that you've assigned undef to $dna at some point or problems with scope. This is a common warning. If you actually want someone to tell you what exactly is wrong you're going to have to show us the code you are running, but you know that already.

As was pointed out above, it basically means that $dna has a value of undef.

The usual "fix" to this is to initialize the variable in question to an empty string (''). If, however, that is not reasonable because the variable is initialized by a function call that could return undef, then the following should be viable:

# using a "modern" perl (5.10 or newer)
my $dna = f(x) // '';

# using an older perl
my $dna = f(x) || '';
[download]

if ( length("$dna") ) {
    ...
}
[download]

if ( length( ($dna || '') ) {
    ...
}
[download]

BTW, You can use diagnostics; in your code to get long-form explanations for such error messages:

Use of uninitialized value in length at cpg8.pl line 4 (#1)
    (W uninitialized) An undefined value was used as if it were alread
+y
    defined.  It was interpreted as a "" or a 0, but maybe it was a mi
+stake.
    To suppress this warning assign a defined value to your variables.

    To help you figure out what was undefined, perl will try to tell y
+ou the
    name of the variable (if any) that was undefined. In some cases it
+ cannot
    do this, so it also tells you what operation you used the undefine
+d value
    in.  Note, however, that perl optimizes your program and the opera
+tion
    displayed in the warning may not necessarily appear literally in y
+our
    program.  For example, "that $foo" is usually optimized into "that
+ "
    . $foo, and the warning will refer to the concatenation (.) operat
+or,
    even though there is no . in your program.
[download]

In order to say more, it'd be useful to see just what line 4 of your script actually says.

Take a look at line 4 and show us the code! :)