in reply to Error meaning

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) || '';

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If worse comes to worst, you can always place the variable inside double quotes to force interpolation into a string for length to work with: 
if ( length("$dna") ) {
    ...
}

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 Edit: Had a brain fart there. This will work: 
if ( length( ($dna || '') ) {
    ...
}

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It helps to remember that the primary goal is to drain the swamp even when you are hip-deep in alligators.

In Section Seekers of Perl Wisdom