Here is a one-liner: 
 perl -naF, -e '$F[0] eq $prev or close FIL,open FIL,qq|>>|, $F[0] . q
+q|.txt|;print FIL $_;$prev=$F[0]'    mycsv.csv

[download]
Where the input file (sorted text file) is mycsv.csv .

If you are on Windows, use double-quotes instead of single.

Update: Changed OPEN to APPEND instead of overwrite, which seems to be what the OP wants.

     ..to maintain is to slowly feel your soul, sanity and sentience ebb away as you become one with the Evil.

In reply to Re: Aggregating Lines from a CSV who lines match a particular field and save those matches based on that matching field name by NetWallah
in thread Aggregating Lines from a CSV who lines match a particular field and save those matches based on that matching field name by Knoperl

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.