for (1 .. 100) {
  # Do something with $_, which will start with the value 1,
  # increasing by one for every loop and stopping with the value 100.
}
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for ('a' .. 'z') {
  # $_ will start with the value 'a', then 'b' all the way up to 'z'
}
for ('aa' .. 'bb') {
  # $_ will start with the value 'aa', then 'ab', then 'ac' up to
  # 'az', then 'ba' and finally 'bb'
}
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while(<>) {
  print if 1 .. 5; #Print lines 1 trough 5 including.
}
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How it works

As any binary operator it takes two operands, the left and the right. It starts by evaluating to false until the left operand becomes true, thereafter it is true until the first iteration after the right operand becomes true, where the state reverts back to false. And then the cycle begins again.
Almost like the && operator, when in the false state, the right operand is not evaluated and, reversed, when in the true state, the left operand is not evaluated
One thing to notice is that when in the false state and the left operand evaluates to true, it will also evaluate the right side in the same operation, just like the && operator, but no matter if the right side is true or false it will return true. If you want to wait until next iteration with checking the right operand for the first time, you can use the .. operators sibling, the ... operator.
An additional feature that we saw already in the first example is that when the operands are constants, Perl will rewrite the code to $. == constant. So that it checks the current line number instead of just the constant.
Examples
Not seldom I find myself having to have a loop variable which keeps track of number of items I've seen within a range. In many cases however I could just extract it from the .. operator, which also tells me how many times it has currently matched.

while(<EMAIL>) {
  my $body = /^$/ .. eof; #First blank line in email is start of body
  my $header = 1 .. /^$/;
  print "looking at line ",int$body," in the body\n" if $body;
  print "looking at line ",int$header," in the header\n" if $header;
}
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Looking for all the paragraphs, with individually enumerated lines in a file? And we don't want the trailing blank line. For that we can use the fact that it returns a "nnnE0" string instead of just the number.

while(<>) {
  my $line = /\S/ ... /^$/;
  print "$line:$_" if $line && $line!~/E0$/;
}
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Conclusion

The .. operator has a lot of little known features and is generally underutilized. I hope you now have a better understanding of what powers it hold.

Now guess what these print
print for ('b'.. 'a')
print for ('a' .. 'B')
print for ('09' .. '08')
print for ('9' .. '8')

T I M T O W T D I