This is my first post so I apologise if its not upto scratch but here is a little something I put together which does what you want (I think). 
@start = (1,2,8,9,10,11,12,43,66,109,121,122,123,657);
@queue = ();
@output = ();

print 'start :: ' , join(',',@start) , "\n";

foreach (@start) {
    if( ($_ - $queue[-1]) == 1 ) {
        # part of sequence
        push @queue, $_;
    } else {
        # store sequence
        push @output , (@queue > 2) ? $queue[0] . '-' . $queue[-1] : @
+queue;
        @queue = ($_);
    }
}

push @output , (@queue > 3) ? $queue[0] . '-' . $queue[-1] : join( ','
+ , @queue );
print 'Expected :: 1,2,8-12,43,66,109,121-123,657' , "\n";    
print 'End :: ' , join(',', @output);

[download]
returns 
start :: 1,2,8,9,10,11,12,43,66,109,121,122,123,657
Expected :: 1,2,8-12,43,66,109,121-123,657
End :: 1,2,8-12,43,66,109,121-123,657

[download]
Hope this helps.

In reply to Re: Number lists to number ranges (was: my head is reeling...) by JamesNewman
in thread Number lists to number ranges (was: my head is reeling...) 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.