The flip flop operator isn't that hard to use. Not in it's simple form anyway. ;) It's ideal spot is for soaking up data from between two identifiable strings. eg

while(<>)
{
    if(/string1/ .. /string2/)
    {
        print;
    }
}
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This will print:

string1
.....
string2
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Note that string1 and string2 remain in the output. shift and pop can help you get rid of them if you don't like them. (Or use splice).

There are far nicer solutions to this problem than using a flip-flop operator, however for demonstrational purposes, this is how I'd solve this using the flip flop. Note that I've added in a third tag to make it more interesting.

#!/usr/bin/perl -w
use strict;

my (@desc, @data, @new);
while(<>)
{
        chomp;
        if(($_ eq "Description:") .. ($_ eq "Data:"))
        {
                push @desc, $_;
                next unless $_ eq "Data:";
        }
                                        # if you have another tag
        if(($_ eq "Data:") .. ($_ eq "NewTag:"))
        {
                push @data, $_;
                next unless $_ eq "NewTag:";
        }
                        # and so on until the last tag
        push @new, $_;
}      
shift @desc;            # get rid of "Description:" from front.
pop @desc if @data;     # get rid of "Data:" as last element.

shift @data;            # get rid of "Data:" from front.
pop @data if @new;      # get rid of "NewTag:" as last element.

shift @new;             # get rid of "NewTag:" from front.

print "desc: @desc\n";
print "data: @data\n";
print "new: @new\n";
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The reason this works (for those people who didn't do or hated electrical or computer engineering) is that the flip flop operator flips to true once the first condition is fulfilled and stays true until the second condition is true where it then flops to false.

So you can have as much gunk above "Description:" as you like and the .. operator will not change. Once "Description:" is seen the .. operator becomes true and the if condition is therefore also true. Once "Data:" is seen the .. operator becomes false (but after the if condition has evaluated to true). If "Description:" should be seen again, then the .. operator would once again flip to true and the if condition would be active.

There are some interesting problems that come up with using the flip flop operator. For starters, if you had another condition in this list:

    if(($_ eq "NewTag:") .. ($_ eq "Somethingelse:"))
    {
        ...
    }
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then it would be possible for two conditions to be active at one time even though only the highest on the list would get any data. This could occur if your file looked like this:

NewTag:            # sets the third flip flop to true
...
Description:        # sets the first flip flop to true
...
Somethingelse:        # is a data line for Description
....
Data:            # now the first flip flop will be
....                    # false and the second true
NewTag:                 # sets second flip flop to false
.....                   
.....            # these get given to the third
.....                   # option finally.
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This of course means that if your data file has missing sets, wierd things will happen. In fact, if a necessary condition does not occur you'll either get too much data or not enough. For example processing a log file with something like:

if(/$date1/../$date2/)
{
}
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should never even be attempted unless you are 300% sure that both $data1 and $date2 will be in that log file and every single other log file you might ever deal with.

I got bitten by that one.

Jacinta