Although dragonchild gives a good example of .., here's a more general explanation, along with the difference between .. and ...

First of all, there is use of .. in a list context:

@a = 1 .. 7;
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Then there's the unrelated use in a scalar context. It only makes sense in a loop.

while (something) {
  if (EXPRSTART .. EXPREND) {
     doit();
   }
}
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is equivalent to:

$inmatch = 0;
while (something) {
  if (!$inmatch && EXPRSTART) {
    $inmatch = 1;
  }
  if ($inmatch) {
    doit();
  }
  if ($inmatch && EXPREND) {
    $inmatch = 0;
  }
}
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Okay? Now for three dots:

while (something) {
  if (EXPRSTART ... EXPREND) {
     doit();
   }
}
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is equivalent to:

$inmatch = 0;
while (something) {
  $wasinmatch = $inmatch;
  if (!$inmatch && EXPRSTART) {
    $inmatch = 1;
  }
  if ($inmatch) {
    doit();
  }
  if ($wasinmatch && EXPREND) {
    $inmatch = 0;
  }
}
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Subtle difference. To see it in action, compare:

while (<>) { print if /A/ .. /B/ }
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with

while (<>) { print if /A/ ... /B/ }
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on this input file

Here's some text before
Some text with an A
some lines in the middle 1
some lines in the middle 2
some lines in the middle 3
Some text with a B
some useless lines 1
some useless lines 2
some useless lines 3
A line with both an A and a B
some lines after the line with both 1
some lines after the line with both 2
some lines after the line with both 3
Once again, text with a B
more useless lines 1
more useless lines 2
more useless lines 3
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