#!/usr/bin/perl
use warnings;
use strict;

my @positions;
my $read_through = 0;

while (push(@positions,tell(STDIN)), defined(my $line = <STDIN>)) {
   print "got line: $line";

   # if the line contains a $ and this is the first time we've
   # got this far in the file, jump back 3

   if (index($line, '$') >= 0 && $read_through < $positions[-1]) {
      $read_through = $positions[-1];
      seek(STDIN, $positions[-4] || 0, 0) or die "whoops, couldn't seek: $!";
   }

   # do stuff with $line

   # only keep positions for last three lines
   splice(@positions, 0, -3, ());
}