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  if ( /[^\x00-\x7f]/ ) # true if $_ contains any non-ascii character

use Encode;

...
if ( $@ ) {
   # Encode would fail/die if $_ was not a valid utf8 string
}

my @bytes = unpack( "C*", $_ ); # break string into bytes

...
die "Incomplete multibyte char\n" if ( $width );

# get here if the string was valid utf8