That could happen if the function didn't already exist before you took a reference from it. Be sure to have loaded the code for that function before you run that snippet. Consider also checking for recursion:

my $old = \&Pod::HTML::scan_podpath;
{
    local $^W;
    *Pod::HTML::scan_podpath = sub {
        if ( $old == \&Pod::HTML::scan_podpath ) {
            croak "...";
        }
        &{$old}( @_ );
    };
}
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