Maybe the problem is elsewhere? Because binmode says:

To mark FILEHANDLE as UTF-8, use :utf8 or :encoding(UTF-8). :utf8 just marks the data as UTF-8 without further checking, while :encoding(UTF-8) checks the data for actually being valid UTF-8.

I read this as that the two should behave identical (except for warnings). Maybe someone else knows where the differences come from.