Variable declaration with a statment modifier can have stange effects, usally a lexical not getting cleared as expected, due to my having both runtime and compile time effects, the test only blocks the runtime effect of resetting the variable. This can be used for some interesting obfu but should be avoided in serious code IMO.

# if EXPR is false $foo will still have the last set value for this sc
+ope'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;
[download]

It's rather common to stick a declaration inside the test of a (BLOCK) loop, often seen with input loops, or looping over both keys and values of a hash at the same time.

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