use strict;

my %actionlist=(
  "you have to read the fourth line" => 4,
  "you have to read the second line" => 2
  );
my $re = "(" . join ("|", keys %actionlist) . ")";
$re= qr|$re|;

  my $printthis = -1;	
  while (<DATA>){
    if (my ($match) = /$re/){
       $printthis = $actionlist{$match};
   }	
   $printthis-- == 0 and print;
  }	  

__DATA__
first line
second line
third line
you have to read the fourth line
one
two
three
four
five
six
you have to read the second line
one
two
three