I suppose that is a buglet, if not a bug. It's not a very important one compared to the handiness of magic opening a pipe from a process and treating it like any other file handle.

Your seekability tests just check that the IO::File object is also in IO::Seekable, which is always true. A better test might look like,

sub IO::File::is_seekable {
    my $self = shift;
    eval { $self->seek( $self->tell()) };
    not $@;
}
[download]

If you had gzipped files instead of zipped, I'd recommend the PerlIO::gzip layer. IIRC, that would make the handle seekable. Unfortunately, I don't know of a PerlIO layer for zip files.