This isn't a code writing service. If you need to use Perl to do this I suggest you learn Perl (http://learn.perl.org, tutorials etc) and use the BioPerl framework if that makes your task easier. I'm pretty sure questions very similar to this have been asked here before Super Search should turn up some results.

In reply to Re: Frequency of amino acids from 'ATOM ' of PDB by marto
in thread Frequency of amino acids from 'ATOM ' of PDB by RRK

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