# if EXPR is false $foo will still have the last set value for this scope's (but not other scope's) $foo
my $foo if EXPR; 

#same here if EXPR is false the first time it's tested on this pass.
my $bar while EXPR; 

##</code><code>##

while (my $foo = <$fh>) {
    #do something
}
while (my ($key, $value) = each %hash) {
    #do other things
}