testfile.txt :

About Apple
Apple is a fruit
I like Apple Juice


Code

open(INFILE,"./testfile.txt")||die ("Cannot open testfile.txt");
open(OUTFILE,">./orange.txt")||die ("Cannot open orange.txt");
while($str=<INFILE>)
{
$str =~ s/Apple/Orange/g;
print OUTFILE $str;
}
close(INFILE);
close(OUTFILE);

Orange.txt


About Orange
Orange is a fruit
I like Orange Juice