If you already have XML::Simple, then you probably already have XML::Parser as well (because the former typically depends on / uses the latter).

So here's a one-liner that does what you want via XML::Parser:

perl -MXML::Parser -e '$p=XML::Parser->new(Handlers=>{Char=>sub{print 
+"$_[1] "}});
   $p->parsefile("filename.xml")'
[download]

(Note that the quotes as shown are based on using a bash shell or equivalent -- not ms-dos/cmd.exe.)

If the amount and types of white-space you get from that are not to your liking, you could either complicate the one-liner a little bit (add tr/ \n/ /s in the sub{}), or just pipe the output to another one-liner...

(updated last paragraph to improve the "tr///" suggestion; to clarify, here's a "really tidy" version of the one-liner:

perl -MXML::Parser -e '$p=XML::Parser->new(Handlers=>
  {Char=>sub{($_=$_[1])=~tr/ \n/ /s; s/^ +$//; print}});
  $p->parsefile("filename.xml"); print "\n"'
[download]

which puts all the visible text on one line, then adds a final line-feed.)