The solutions offered so far are very sophisticated, but I think the most stupid way is also the most readable:

my %h_prefixed;
my %h_numbers;

while(my ($key, $val) = each %h) {
    $h_numbers{$key} = $val if looks_like_a_number($val);
    $h_prefixed{$key} = "p$val";
}
[download]

And it's (probably) not even slower or anything.

For a (maybe a bit) less readable solution, I'd like to offer the following:

my %h_prefixed;
@h_prefixed{keys %h} = map "p$_", values %h;
[download]