then go for an external sort and do the duplicate consolidation as a postprocessing: 
open my $fh, '-|', 'sort', $filename or die;
chomp(my $line = <$fh>);
my ($prev_key, $prev_value) = split /,,/, $line;
while (<$fh>) {
  chomp;
  my ($key, $value) = split /,,/;
  if ($prev_key eq $key) {
    $prev_value .= $value;
  }
  else {
    print "$prev_key,,$prev_value\n";
    ($prev_key, $prev_value) = ($key, $value);
  }
}
print "$prev_key,,$prev_value\n";

[download]

Read the manual page for your OS sort utility in order to find out how to optimize its usage.

Or you could also use Sort::External.

In reply to Re^3: perl ST sort performance issue for large file? by salva
in thread perl ST sort performance issue for large file? by rkshyam

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.