I was just writing some code and used:

{ msg => $IN->param('msg'), some_tag => $output, other_tag => $bla }

.....this caused a big screw up on the page and some_tag and other_tag were "unknown".

It turns out I had to use

msg => scalar $IN->param('msg')

Why did I need to do that?.....msg = 1 and I've used it without the scalar on other templates .