Check your conditionals and make sure nothing is executing that you think isn't. I just copied your above code:

use CGI qw/:standard/;
$thisscript = 'foo';

print p("line 109");
print start_form({ -action  => "$thisscript",
        -method  => "post" });
print hidden('action','logout');
print submit('Logout');
print end_form;
print p("line 115");
[download]

When I ran, I got the expected result:

<p>line 109</p><form method="post" action="foo" enctype="multipart/for
+m-data">
<input type="hidden" name="action" value="logout"  /><input type="subm
+it" name="Logout" value="Logout" /></form><p>line 115</p>
[download]

So something is running that you think isn't.