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AB   Naja atra (Chinese cobra).
OC   Eukaryota; Metazoa; Chordata; Craniata; Vertebrata; Euteleostomi;
...
FT   CHAIN        22     81       CYTOTOXIN 2.
FT   DISULFID     24     42
//

perl myprog.pl a.db

$/ = "\/\/\n";
$counter=1;
...
  #do something  
    $counter++
    }

my $infile =$ARGV[0];
open(IN, $infile) or die "Can't open input file: $!";
...
  # do something
    $counter++
    }