sub rndStr{ join'', @_[ map{ rand @_ } 1 .. shift ] };;

$x = rndStr( 80, qw[a c g t] );;

print length $x, ':', $x;;
80 : actaatcttgcgccgcggcttcatacgagatgaatagtacgaaaacttggatacacctgtatcatagaagggccgctgcg