Here's one way - construct another hash: 
#!/usr/bin/perl

use strict;
use warnings;
use Data::Dumper;

my %hash = map {split /\s+/} <DATA>;

my %new_hash;
for my $key (keys %hash){
  if ($key =~ /213119_at/){
    $new_hash{$hash{$key}}++;
  }
}

print Dumper \%new_hash;

__DATA__
Affy:HG_U133A:213119_at:74:303;         ENSG00000123643
Affy:HG_U133A:213119_at:542:439;        ENSG00000123643
Affy:HG_U133A:213119_at:658:369;        ENSG00000123643
Affy:HG_U133A:213119_at:199:255;        ENSG00000123643
Affy:HG_U133A:213119_at:436:453;        ENSG00000123643
Affy:HG_U133A:213119_at:324:381;        ENSG00000458158
Affy:HG_U133A:213119_at:584:557;        ENSG00000123643
Affy:HG_U133A:213119_at:234:507;        ENSG00000123643
Affy:HG_U133A:213119_at:482:429;        ENSG00000123643
Affy:HG_U133A:213119_at:608:451;        ENSG00000458158
Affy:HG_U133A:213119_at:356:297;        ENSG00000123643

[download]
output: 
---------- Capture Output ----------
> "C:\Perl\bin\perl.exe" _new.pl
$VAR1 = {
          'ENSG00000123643' => 9,
          'ENSG00000458158' => 2
        };

> Terminated with exit code 0.

[download]
note: there are 9 :-)

In reply to Re: Counting hash values by wfsp
in thread Counting hash values by MonkPaul

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.