As moritz said, 1 (or ++) is a common choice of value, but it's not necessarily the best one. It doesn't usually matter these days for 100k elements, but it's better (thanks, Liz! ;) for memory size to do something like this:

my ($undef);
while (whatever...) {
    ....
    $hash{$key} = $undef;
}
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This way each element points to the same $undef value. Otherwise, each element would point to a different copy of the value 1. That's a kind of "poor man's aliasing". For bonus points, you might look at Array::RefElem or Data::Alias.