What's wrong with always quoting "$vars"?

The problem is that those double-quotes force stringification, coercing numbers and references into strings, even when you don't want them to be.

If you get used to writing odd things like these: 

    print "$var";   	# BAD
    $new = "$old";   	# BAD
    somefunc("$var");	# BAD

You'll be in trouble. Those should (in 99.8% of the cases) be the simpler and more direct: 

    print $var;
    $new = $old;
    somefunc($var);

Otherwise, besides slowing you down, you're going to break code when the thing in the scalar is actually neither a string nor a number, but a reference: 

    func(\@array);
    sub func {
	my $aref = shift;
	my $oref = "$aref";  # WRONG
    }

You can also get into subtle problems on those few operations in Perl that actually do care about the difference between a string and a number, such as the magical ++ autoincrement operator or the syscall function.