Sometimes I wanted to do that but I didn't know what the syntax is.

There are several ways to do that. Here are two more.

my ($foo, $bar, $baz) = map "default", 1..3;
$_="default" for my ($foo, $bar, $baz);
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As obscure as the last one is, I'd prefer it over the others if you think that the variable list will expand since you don't have to synchronize the number of variables with a hard-coded constant.

That is, if I wanted to achieve this effect. Which I never have needed to.

$_="default" for my ($foo, $bar, $baz);

That one surprises me that it works. I thought that the "for" statement modifier was mere syntactic sugar, since that deparses as:

u2-4.14 $ perl -MO=Deparse ./tst   
foreach $_ (my($foo, $bar, $baz)) {
    $_ = 'default';
}
./tst syntax OK
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But if written as a straight for loop (as it deparses to), then the variables are local to the loop. And I could've sworn I tried that once upon a time, and it didn't work...probably won't be the last time I make that mistake :)