If you have multiple lines in a field (between the commas), the data needs to be in quotes so you can match it with a regep if you want to manipulate just it and leave everything else alone. Or if you need to mess with all of the fields you can split on commas and get an array to work with.
Its hard to answer your question, since you never really say what you want do with the last line.
This will get you all of the values into an array

 
@fields = split /,/$your_line_here;
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If you want to mess with the last field more you can match say $fields[@fields-1]
Or if you want to just mess with the last field and nothing else try this...

$foo =~ /,\"(.+)\"/;
$bar = $1;
...do some stuff....
$foo =~ s/,\".+\"/,\"$bar\"/;
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The regexp could use work... but thats your job.
ps, dont you loose the bullets when you take the file out of .xls (I am assuming the file was created from excel, but I could be wrong :) )?