my $line = "desc\tdate\tdetails"; # This will be the newline
@ARGV = "file.tab"; # Init input file
$^I = ".bak"; # Inplace editing with a bak up file

if (-z "file.tab") {
# somehow set the file up with a fake line to chew on ???
}

while (<>) {
# if theres no more lines after
if (eof(ARGV)) {
# take a slice of the last line
# the id number to be precise
# add 1 to get the next id number for
# the newline we're inserting
# also put the rest of the line on the end
# and piggy back the line onto the last line of the file
$_ .= (split /\t/)[0] + 1 . "\t$line\n";
}
print;
}