I'm not terribly proficient myself, but I think this will work:
/test.txt:

first,line,and,end
second,line,with,a
break
third,line,no,break
[download]

The code:

#!/usr/bin/perl -w
use strict;
my @array;
my @bigun;
my $i;
open (FILE, "/test.txt");
while (<FILE>){
   chomp($_);
   if (/,/){
      #out with the old
      push @bigun, (join ' ',@array) if (@array);
      #and in with the new
      @array=split /,/,$_;
   }
   else {
      #append the 'un-commaed' line to the lest element of the array
      $array[$#array]=$array[$#array].$_;
   }

}
push @bigun, (join ' ',@array);
foreach $i(0..$#array){
   print "$bigun[$i]\n";
}
[download]

And, the output:

first line and end
second line with abreak
third line no break
[download]

It's kind of dirty, but it'll do the job..
Marty

update: added use strict;. Mas apologies for that....