I have written a script to grap the duplicate row of a file. 
1 #!/usr/bin/perl -w
  2 use strict;
  3 my %dup;
  4 while(<>){
  5 my @row=split(/\s+/,$_);
  6 $dup{$row[3]}=$dup{$row[3]}+1; #if change to $dup{$row[3]}++, it w
+ill run smoothly no error report
  7 }
  8
  9 foreach my $id (keys %dup){
 10 if ($dup{$id}>1) {
 11 print "$id\t$dup{$id}\n";
 12 }
 13 }

[download]
it report the error: "Use of uninitialized value in addition (+) at dup.pl line 6.." If I change that line code to "$dup{$row3}++", then it's all right. Who can tell me why this happened? Thanks.

In reply to why such an error? by lightoverhead

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.