From type/IO/Path

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method s(IO::Path:D: --> Int:D)

Returns the file size in bytes. May be called on paths that are directories, in which case the reported size is dependent on the operating system. The method will fail with X::IO::DoesNotExist if the path points to a non-existent filesystem entity.

say $*EXECUTABLE.IO.s; # OUTPUT: «467␤»

From type/IO/Special

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method s(IO::Special:D: --> 0)

The 'size' file test operator, always returns 0.