my $foo;  # Predeclared: bad style

# Usually done by C-coders, who predeclare a whole bunch of variables:
# my ($foo, $bar, $baz, $quux, $whatever, $i, $think, $is, $needed, $anywhere);

...

$foo = foo();

...

print $foo;

##</code><code>##

my $foo;

...

print $foo;  # No error, just a warning at run time.

##</code><code>##

my $foo = foo();  # Declared when needed

...

print $foo;

##</code><code>##

...

print $foo;  # Error at compile time!