use strict;
use warnings;
use Test::More qw(no_plan);

#create a.txt contining "a"
open F, "> a.txt";
print F "a";
close F;

#fails because no substitution was made
#prints to STDOUT
process_file(); 


local $^I = ".bak";

#now it works.
#prints to "a.txt" as we want. But how does it know to print there?
write_and_read_file(); #ok

sub process_file {
	
	@ARGV = qw(a.txt);
	
	local $/; # slurp it
	#changes a.txt so contains "b"
	#the diamond operator operates on @ARGV by default
	while (<>) {
		#$_ now contains the entire file, alter at will
		s/a/b/g;
		#How does the print operator know what file handle to print to? 
		#Was this selected somewhere by magic?
		#Where is this documented / explained?
		print;
	}

	#need to reset @ARGV, I guess it got wiped out somewhere in that while.	
	@ARGV = qw(a.txt);
	while (<>) {
		is($_,"b");
	}
	
}