$bar = "Hello %s world!\n";

if ($bar =~ m/(Hello)/) {
	$foo = lc $1;
	$foo =~ s/h/j/;
	printf $bar, $foo; # prints "Hello jello world!"
}

##</code><code>##

$bar = "Hello $foo world!\n";

if ($bar =~ m/(Hello)/) {
	$foo = lc $1;
	$foo =~ s/h/j/;
	print "$foo\n";	# prints "jello"
	print $bar;		# prints "Hello  world!"
}