For tasks like you've described, there's no reason to read the entire file into an array. It's usually better to process the file line by line, because it scales in case your files should get huge. I.e. for, say, a 1 GB file, you'd need several gigs of RAM to hold the contents in the array, while if you process line by line, you'll only ever need storage for one line. 

open my $fh_in,  "<", $proteinfilename or die "Couldn't open '$protein
+filename': $!";
open my $fh_out, ">", $outfilename or die "Couldn't open '$outfilename
+': $!";

while (my $line = <$fh_in>) { 
    if ($line =~ /^AD/) {
        print $fh_out " $line";
    }
}

[download]

In reply to Re^3: String related question by Anonyrnous Monk
in thread String related question by vikuuu

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.