You are getting warnings about uninitialized values because you are walking off the end of your arrays (I think). Am I correct in thinking you'd like to systematically check all Accession numbers against your GI list? To do that, you need to iterate over both lists, which in general means you need two loops - one for GI and one for Accession. If you also use last (assuming max 1 hit per target) to do some Loop Control, you might code something like: 
for my $Accession (@Accession) {
    for my $GI (@GI) {
        if ($Accession =~ $GI) {
            print "$GI\n";
            last;
        }
    }
}

[download]
Note I've used the Foreach Loops construct instead of using an explicit index because you do not actually care for your output what the index was.

#11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.

In reply to Re: Using regular expressions with arrays by kennethk
in thread Using regular expressions with arrays by andybshaker

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.