while (my ($var, $cnt) = each %hash) {
    push @flat, ($var) x $cnt;
}
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Unlike with the respective map solution, this allows you to use self-documenting variable names, and it also scales better performance-wise (both memory and execution time).  (Some people still prefer to use map — presumably because it allows them to show they've grasped basic functional programming concepts (or so, I guess).)