Why does Perl think -1 is true?

This is a piece of common example code:

while (1) { 
    print "foo\n"; 
}

which prints 'foo' forever.

perl foo.pl
foo
foo
foo
...

and

while (0) { print "foo\n"; } 

dies quietly as you expect:

perl test.pl

Can someone explain why this is a useful implementation of while? This works on 5.10 at least, Unix and MacOS X:

while (-1) { print "foo\n"; }

which gives

foo
foo
foo
...