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

my $fn = \&doOtherProcessing;
while (<DATA>) {
    if (/for/)   {$fn = \&doStage1}
    if (/their/) {$fn = \&doStage2}
    if (/fruit/) {$fn = \&doStage3}
    if (/lazy/)  {$fn = \&doOtherProcessing}
    $fn->($_);
}

sub doStage1            { print "1: ",shift }
sub doStage2            { print "2: ",shift }
sub doStage3            { print "3: ",shift }
sub doOtherProcessing   { print "*: ",shift }

__DATA__
now is the time
for all good men to
come to the aid of
their party.
Time flies like an arrow
fruit flies like a  banana.
The quick red fox
jumps over the lazy
brown dog.
Etaoinshrdlu.

##</code><code>##

*: now is the time
1: for all good men to
1: come to the aid of
2: their party.
2: Time flies like an arrow
3: fruit flies like a  banana.
3: The quick red fox
*: jumps over the lazy
*: brown dog.
*: Etaoinshrdlu.
*: