$ perl -e " print chr(999) "
Wide character in print at -e line 1.
ϧ
$ perl -e " binmode STDOUT; print chr(999) "
Wide character in print at -e line 1.
ϧ
$ perl -Mdiagnostics -e " print chr(999) "
Wide character in print at -e line 1 (#1)
    (S utf8) Perl met a wide character (>255) when it wasn't expecting
    one.  This warning is by default on for I/O (like print).  The eas
+iest
    way to quiet this warning is simply to add the :utf8 layer to the
    output, e.g. binmode STDOUT, ':utf8'.  Another way to turn off the
    warning is to add no warnings 'utf8'; but that is often closer to
    cheating.  In general, you are supposed to explicitly mark the
    filehandle with an encoding, see open and perlfunc/binmode.

ϧ
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So to silence the warning, use

$ perl -e " no warnings q[utf8]; print chr(999) "
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using bytes would also silence it, but would change the semantics

$ perl -e " use bytes;  print chr(999) "
τ
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perluniintro, perlunicode