How bent are you on testing that the resulting test matches exactly? Wouldn't it be enough to match that the resulting error message contains

02-internals_magic.t at line ...
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I would use a regular expression to confirm that the string ends with what you want, instead of trying to cater for every native file system name. Also, maybe you want to avoid File::Spec if you want Unix-semantics filenames. For unixish semantics (as Perl uses in its error messages), you can always concatenate with slashes.