in reply to Base sequence length in fasta format file

By base i assume you mean each occurence of the four letters representing A is for adenine, G is for guanine, C is for cytosine, T is for thymine, and so you would want to count each of those occurences? so you could
while(<input>) { $sequence = $_; chomp($sequence); $count=length($sequence); if ($count >= $ideal) { print "$sequence\n"; # or whatever you want here } }

Hope this is what you had in mind and that it helps
"when a new client is created, we have to kill all the children..." --Sams Teach yourself Perl 5