While that and works, it also undefines @foo, %foo, etc. It seems to me that the OP is trying to undefined &foo specifically.

Other working solutions with the same caveat:

my $x = \*foo;
undef *$x;
[download]

and

my $x = 'foo';
{
   no strict 'refs';
   undef *$x;
}
[download]

and

my $x = do { no strict 'refs'; \*{'foo'} };
undef *$x;
[download]