OK, now I know the answer is almost always "No!", but I'm at the end of my tether, and short of diving down into C code, I can't take it any further.

I'm using perl, v5.8.0 built for i386-linux-thread-multi.

Essentially it seems that UNIVERSAL::isa, my favourite swiss army knife function, is behaving weirdly. Here is the most simple example I can boil it down to.
Loading DB routines from perl5db.pl version 1.19
Editor support available.

Enter h or `h h' for help, or `man perldebug' for more help.

main::(-e:1):   1
  DB<1> $Foo::VERSION = 1;

  DB<2> sub Foo::new { bless {}, shift }

  DB<3> x UNIVERSAL::isa( 'Foo', 'UNIVERSAL' );
0  1
  DB<4> x UNIVERSAL::isa( 'Foo', 'UNIVERSAL' );
0  1
  DB<5> x UNIVERSAL::isa( 'Foo', 'UNIVERSAL' );
0  1
  DB<6> @Foo::ISA = ();

  DB<7> x UNIVERSAL::isa( 'Foo', 'UNIVERSAL' );
0  1
  DB<8> x UNIVERSAL::isa( 'Foo', 'UNIVERSAL' );
0  ''
  DB<9> x UNIVERSAL::isa( 'Foo', 'UNIVERSAL' );
0  ''
  DB<10>

[download]
The particular problem I have with this situation is that, if your class doesn't have an @ISA, lots of things ( overload and Carp::croak for example), accidentally set @SomeClass::ISA to (), when doing things like this

Example from overload::mycan 
  foreach $p (@{$package . "::ISA"}) {

[download]

This breaks my usual "is this an object of some sort" test of UNIVERSAL::isa( ref $Object, 'UNIVERSAL' ).

What do you all think?

In reply to Have I found a bug in Perl? by adamk

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.