my $str="17:43:33:21:23:19:27:6";
my @aux = split /:/, $str;
my @ary = map {($aux[2*$_+1]) x  $aux[2*$_]} 0 .. @aux/2-1;
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UPDATE:
You can try the following code also:

my $str="17:43:33:21:23:19:27:6";

my %hash = split /:/, $str;

my $adno;

my $rand = rand 100;

my $sum = 0;
for (keys %hash) { # there is no need of sorted keys
    $adno = $hash{$_};
    last if ($sum += $_) > $rand;
}

print $adno;
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UPDATE: sort keys eliminated in 'for'