use strict;
use warnings;

print "This is test perl program\n";

open ( FILE1,"<1.txt") or die "cannot open $?:$!";

my ($test1, $test2);

while (<FILE1>){
chomp ($_);
my ($i) = $_ =~ s/apple/APPLE/;
#just replacing apple with APPLE
#this works fine
if($i)
{
$test1=$_;
} 
}

close FILE1;
print "test1 = $test1\n\n"; 
#this has the the whole line which is of #the form a b

my ($a,$b)=split(/\s/,$test1,2);
print "b = $b\n";

open (FILE2, "<2.txt") or die "cannot open $?:$!";
while (<FILE2>){

chomp($_);

my ($i) = $_ =~ s/$b/MANGO/;

if($i)
{
$test2=$_;
} 
}

close FILE2;
print "\ntest2 = $test2\n\n";
my ($c,$d)=split(/\s/,$test2, 2);
print "d = $d\n";

outputs:
--------
This is test perl program
test1 = APPLE mango

b = mango

test2 = MANGO pineapple

d = pineapple