sub foo {
  print "yep\n";
}

@foo = $foo = 'a string';
my $f = 'foo';

foo() if defined &foo;

undef &$f;

print "nope\n" unless defined &foo;

print "\$foo: $foo\n";
print "\@foo: @foo\n";

__output__

yep
nope
$foo: a string
@foo: a string