This does most of the work for you, but I've left some things for you to do:

<DATA>;

my @prev_line;
while ( <DATA> ) {
    my @this_line = split /[,\s]+/;
    shift @this_line;
    print join( ", ", map { "$this_line[$_]-$prev_line[$_]" } 0 .. $#t
+his_line ), "\n"
        if @prev_line;
    @prev_line = @this_line;
}
__DATA__
date, val1, val2, val3, val4
1/2/2007, 1, 4, 5, 6
1/3/2007, 2, 5, 7, 10
1/5/2007, 5, 6, 8, 11
[download]

Output:

2-1, 5-4, 7-5, 10-6
5-2, 6-5, 8-7, 11-10
[download]

You may find this version easier to follow (using the same __DATA__ as earlier):

<DATA>;

my @prev_line;
while ( <DATA> ) {
    my @this_line = split /[,\s]+/;
    shift @this_line;

    if ( @prev_line ) {
        my @output;
        for ( 0 .. $#this_line ) {
            push @output, "$this_line[$_]-$prev_line[$_]";
        }
        print join( ", ", @output ), "\n";
    }
    @prev_line = @this_line;
}
[download]

That leaves you with the job and handling the files.

I'm sure google will help, and there's always the Perl documentation.

update: omitted __DATA__ first time round
update^2: added second example

---

---

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! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

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:
	&		&
	<		&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.