Not much optimization here, but provides a good starting point who want to take it further.

my $hash = { 1 => 0.2,
         2 => 0.3,
         3 => 0.5
         };

my $sum;
my $hash2;
while(my($key,$value) = each %{$hash}){
    $sum += $value;
    print "$sum . $key\n";
    $hash2->{$sum} = $key;
}

foreach (1..10){
    my $x =   rand(1);
    print "$x\t";
    foreach (sort { $a <=> $b } keys %{$hash2}){
    if($x < $_){
        print $hash2->{$_},"\n";
        last;
    }
    }
}
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