use strict;
use warnings;


sub main {

	my $input = ('D:\Perl\Paul\My_Text.txt');
	my $output = 'My_Output.txt';
	
	open(INPUT, $input) or die("$input - Not found \n");
	open(OUTPUT, '>'.$output) or die("Can't create $output \n");
	
	while(my $line = <INPUT>){			
			 if($line =~ s/\bfox\b/dog/ig){
				# The \b above are word boundaries
				# the ig at the end sets "case insensitive" and search globally
				
				print OUTPUT $line;
				
			 }
		
		}
		
	close(INPUT);
	close(OUTPUT);
	
}
	

main();