First I'd like to slap you with a customary RTFM, since the first page of the IO::InnerFile documentation states: "Note that FILEHANDLE must be able to seek() and tell(), in addition to whatever other methods you may desire for reading it." ;)

broquaint offered a solution that manipulated IO::Handle's @ISA. Whilst this works I wouldn't recommend it. Making IO::Handle provide IO::Seekable's methods would perhaps break another class that inherits IO::Handle and uses AUTOLOAD. Or maybe a class inherits something more than IO::Handle, and that other class implements IO::Seekable's methods.

I suggest making an own class that inherits both IO::Handle and IO::Seekable,

  {
    package MyHandle;
    use base qw/ IO::Handle IO::Seekable /;
  }
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and then bless the filehandle into that, before passing it to &IO::Innerfile::new:   bless $fh => MyHandle::; Now $fh implements what's needed to make IO::InnerFile function.

Hope I've helped,
ihb