You need to check that $accession{$g} exists before using it. Something like changing the line

 if ($accession{$g} =~ m/$accession_id/i){
to 
 if (exists $accession{$g} && $accession{$g} =~ m/$accession_id/i){

should do the trick.


EDIT: BrowserUK posted an answer while I was composing mine. His is more comprehensive.

In reply to Re: fasta file by molecules
in thread fasta file by PrincePerl

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