To find out if your username exists in the database, you need to make a SELECT query to the database asking for any records with $username in them.

I would suggest that you read some of the tutorials here on DBI:

(I'm refraining from answering the question directly because I believe you'll find learning the answer from the tutorials more rewarding in the long term.) :)

I would also suggest that you look into using placeholders in your query. Right now, you're interpolating the strings '$username' etc. into your insert statement. While this will work most of the time, someone could enter "T'Pau" as a username, and your insertion statement would be: 

...Values ('T'Pau'...

[download]
which would then be a database error, since the quotation would end after the T. At very least, I'd change it to the following: 
 
$query = qq(insert into members (username, password, email, name, url,
+ date) Values (?, ?, ?, ?, ?, ?));
$sth = $dbh->prepare($query);
$sth->execute($username, $password, $email, $name, $url, $time) or die
+ "Error in execution: $DBI::errstr";

[download]
You can learn more about this in the abovementioned tutorials.

stephen


Update: Added explanation of indirect answer.

In reply to Re: mySQL with Perl by stephen
in thread mySQL with Perl by skirrow

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.