You are assigning the value 0 to every key so every key will be defined however you can't test for truth because 0 is false. If instead of assigning 0 you assigned any non-zero value you wouldn't have to use defined() to test the keys: 
my $xpid = 1234;

%procs = map { $_ => 1 } @{ $dbh->selectall_arrayref( "select spid fro
+m v\$process" ) };

if ( $procs{ $xpid } ) {
   print "Pid is in use : $xpid\n";
}

[download]
Another way to do it: 
my $xpid = 1234;

@procs{ @{ $dbh->selectall_arrayref( "select spid from v\$process" ) }
+ } = ();

if ( exists $procs{ $xpid } ) {
   print "Pid is in use : $xpid\n";
}

[download]

In reply to Re^2: search an array by jwkrahn
in thread search an array by Anonymous Monk

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.