The following code should illustrate the difference:

#!/usr/bin/perl

$_ = "Foollll\nFoollll\n";
1 while s/^Fool/Foo/g;
print "1:\n\n$_";

print "\n\n";

$_ = "Foollll\nFoollll\n";
s/^Fool/Foo/g;
print "2:\n\n$_";
[download]

Can you see what the output will be...? ;)