A simple and efficient method is to build an array of items to pick where each item is replicated the number of times to match it's weighting. (Note: I factored the weights by 5 as that reduces the size of the array.);

Then you just pick (once!) from that array and your guaranteed to get darn close to your chosen weightings:

#! perl -slw
use strict;
use List::Util qw[ sum ];

our $N ||= 1e3;

my %group = ( ad1 => 1, ad2 => 2, ad3 => 4 );

my $t = sum values %group;

print "Expected";
for ( sort keys %group ) {
    printf "$_ to be chosen %f%%\n", $group{ $_ } / $t * 100;
}



my( $key, $value, @choices );
push @choices, ($key) x $value while ( $key, $value ) = each %group;

my %picks;
for ( 1.. $N ) {
    $picks{ @choices[ rand @choices ] } ++;
}

print "\nActual";
for ( sort keys %picks ) {
    printf "$_ chosen %f%%\n", $picks{ $_ } / $N * 100;
}

__END__
C:\test>700683 -N=1e6
Expected
ad1 to be chosen 14.285714%
ad2 to be chosen 28.571429%
ad3 to be chosen 57.142857%

Actual
ad1 chosen 14.276200%
ad2 chosen 28.601600%
ad3 chosen 57.122200%

C:\test>700683 -N=1e6
Expected
ad1 to be chosen 14.285714%
ad2 to be chosen 28.571429%
ad3 to be chosen 57.142857%

Actual
ad1 chosen 14.309300%
ad2 chosen 28.577100%
ad3 chosen 57.113600%
[download]