$/ = "";
while(<DATA>){
    s#=[^\n\S]*\n(?!.*=)(?!</template>)#= #g;
    print;
}
__END__
<template>
  foo                = Yes
  bar                = 0
  blort              = 
  home_url           =
http://www.foo.com/some/really/long/url 
  zoo                = xxx
  _url               =
http://www.foo.com/some/really/long/url 
  boo   =  
</template>