I can't possibly imagine any use of calling my in scalar context

if (my $x = ...) { ... }
open(my $fh, ...)
read(..., my $buf, ...)
my $r = \my $s;
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Perl already has the disadvantage to default to global vars while other languages automatically restrict to a local scope

What language are you talking about? C, C++, Java, JavaScript and VB all require local declarations.

a warning should be emitted when using a my(LIST) in scalar context.

More precisely, my LIST with a list of more than one variables.

Because of its side-effects, my LIST could legitimately be used in scalar context. This job may be more appropriate for a linter (Perl::Critic). But since that use is rare and easy to rewrite (into something clearer), a warning could be appropriate. Feel free to create a ticket (just need to email perlbug@perl.org).

I only looked quickly, but my and friends might be the only builtins to return something of questionable use in scalar context.