There's a function in Module::Locate that determines whether a scalar acts like a filehandle or not. So with that you could do something like this in your function

use Module::Locate 'acts_like_fh';

sub fizbin {
  my($str, $infh, $outfh) = @_;

  -e $_ or acts_like_fh $_
    or die "Invalid file name/handle '$_'"
      for $infh, $outfh;

  ...
}
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