Here is an example using the first line of data compared to the other values. Note, it skips lines containing ! which may precede the first data line

#!/usr/bin/perl
use strict;
use warnings;

my $file = $ARGV[0];
open (RAW, '<', "./$file") or die "Can't open $file for read: $!";

my @first;
my $line_count = 0;
while (<RAW>){
  chomp;
  next if /!/;
  my @col = split /,/,$_;
  if (++$line_count == 1){
    @first = @col;
    print "\n--------------------------------\n";
    print "Start $first[1]\n";
    print sprintf "%10s %10s %10s\n",'+secs','value','change';
    print sprintf "%10s %10s %10s\n",'-'x10,'-'x10,'-'x10;
  }
  print sprintf "%10d %10.3f %10.3f\n",
    ($col[0] - $first[0]),
     $col[2],
    ($col[2] - $first[2]);
}
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Outputs

--------------------------------
Start  2015-02-22 16:15:00
     +secs      value     change
---------- ---------- ----------
         0   4294.700      0.000
        60   4289.700     -5.000
       120   4299.800      5.100
       180   4302.800      8.100
       240   4296.900      2.200
       300   4301.000      6.300
       360   4300.300      5.600
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