My actual program has some useless lines before the ones I posted, along the lines of use warnings; use strict; (yes, I use them always). Under that conditions:

$ perl -Mstrict -wle 'print "$_: " . ($! = $_) for @ARGV' 13 20 invali
+d
13: Permiso denegado
20: No es un directorio
Argument "invalid" isn't numeric in scalar assignment at -e line 1.
invalid:
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In fact, even with the grep, a bug remains where the parameter 0 is supplied. I didn't bother fixing it.