Thats rather clever.... There is a subtle issue with extrapolating that particular idiom though. Suppose we wanted to match lines of exactly four fields, but were only interested in the first two fields of such lines:

dog:cat:pig       <== skip, only has three fields
Amy:Ann           <== skip, only has two fields
ape:ant:bug:car   <== match this line (it has four fields)
                      but we only need to keep 'ape' and 'ant'
[download]

Tweaking the code above, we might think that this will do the trick for us:

while (<DATA>) {
    chomp;
    (my ($first, $second) = split /:/) == 4 or next;
    print "$first $second\n";
}

__DATA__
dog:cat:pig
Amy:Ann
ape:ant:bug:car
[download]

However, that doesn't seem to work for us, nothing gets printed. In fact, try as we might, we can't construct any line at all that passes the IDIOM==4 test.

So, does anyone want to guess why IDIOM==4 is so different from IDIOM==2?

Update: As an added hint, testing for IDIOM==3 will match the 'dog:cat:pig' line....

-Blake