I've never see assigning to an empty list used to force a list context. Assigning to an empty list is used to count the number of elements in a list (which has nothing to do with context).

It has to do with context. Assigning to the empty list forces list context upon the RHS. It returns the empty list in list context, and the number of elements of the RHS in scalar context. Consider:

print my $x = ( 'a', 'b', 'c' );         # c
print my @x = () = ( 'a', 'b', 'c' );    # nothing
print my $x = () = ( 'a', 'b', 'c' );    # 3
print my $x = @a = ( 'a', 'b', 'c' );    # 3

print scalar ( 'a', 'b', 'c' );          # c
print +() = ( 'a', 'b', 'c' );           # nothing
print scalar ( () = ( 'a', 'b', 'c' ) ); # 3
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update: to make it clearer...

sub bar {
    local $\;
    print "bar:";
    print wantarray ? "list - " : "scalar - ";
    return ( "d", "e" );
}
sub foo {
    local $\;
    print "foo:";
    print wantarray ? "list - " : "scalar - ";
    return ( "c", bar );
}
$\ = "\n";

print ( 'a', 'b', foo );                 # adcde
print scalar ( 'a', 'b', foo );          # e
print +() = ( 'a', 'b', foo );           # nothing
print scalar ( () = ( 'a', 'b', foo ) ); # 5
__END__
foo:list - bar:list - abcde
foo:scalar - bar:scalar - e
foo:list - bar:list - 
foo:list - bar:list - 5
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