Well, here's one way, but I don't know how efficient it is:

#!/usr/bin/perl -w
use strict;

my %probs = (
 'fifty'  => 50,
 'ten' => 10,
 'forty' => 40,
);

# generate lookup for the above
my @lookup;
foreach my $key (keys(%probs))
{
 for (1..$probs{$key})
 {
  push @lookup, $key;
 }
}

# now pick one:
sub pickOne
{
 $lookup[ int(rand($#lookup)) ];
}

# test it

my %bins;
for (1..1000)
{
 $bins{ pickOne() }++;
}

for my $key (keys(%bins))
{
 print $key, "\t", $bins{$key}/10, "%\n";
}
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