well, i'm one of those people that considers warnings blasphemy, so i avoid them like the plague. In a case like this I'd simply use something like:

print "some sort of introduction of the following comes here";
if($var1) { print "\$var1 = $var1 "; }
if($var2) { print "\$var2 = $var2 "; }
if($var3) { print "\$var3 = $var3"; }
print "\n";

perhaps i'm allowing the quirks of the language to dominate my style here, but I have all my bases covered this way :-)

Additionally, I tend to encapsulate a construction like this in a if($debug) {} so as to allow me to turn all the debug messages in a script on or off with the change of a single variable somewhere.