You can save a single character by using a glob, such as:

sub o {
($l,*w,$")=@_;sub g{$_=pop;s/./chr index"@$l",$&/ges;$_}sort{g($a)cmp 
+g+$b}@w
}
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So *w ... @w instead of $w ... @$w.

'1e9' or 'chr' vary only in that '1e9' would support an alphabet of length > 255, but if you're using UNICODE, then chr would likely return two characters anyway.