Why wouldn't that be a syntax error?

Because ?: can be used on either the right hand side of an assignment:

my $var = $test ? $value_one : $value_two;
[download]

Or on the left hand side of an assignment:

( $test ? $var_one : $var_two ) = $value;
[download]

And because $pkg->{'DIST'} can be assigned to it is not a syntax error.