If you want to use wantarray to tell you the context of an entity within a larger expression (which is what you seem to be trying to do), you have to call wantarray from within a sub, inserted into that larger expression.

So the following two examples will do what you're seeking:

print bleah(), "\n";

sub context {
    return wantarray() ? "List" : "Scalar";
}

sub bleah {
    return ( "Context: " . context() );
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...or if you're trying your hand at obfu...

print bleah(), "\n";

sub bleah {
    return ( "Context: " 
             . &{sub { wantarray() ? "List" : "Scalar" }}() );
}
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The second example creates an anonymous sub in which wantarray may reside, and then dereferences the anonymous sub, executing it within the concatenation expression.