#! perl
use strict;
use warnings;
use Data::Dumper;
use constant { WELL => 0,        SICK   => 1,    };

my  @pop = ( { name => 'Fred',   status => WELL, },
             { name => 'Wilma',  status => WELL, },
             { name => 'Barney', status => WELL, },
             { name => 'Betty',  status => WELL, }, );

(my $pop = scalar @pop) > 0                    or die "Empty population\n";
print "How many random infections? (Enter a number between 1 and $pop):\n";
chomp(my $answerinfect = <>);
($answerinfect > 0) && ($answerinfect <= $pop) or die "Invalid selection\n";

for (1 .. $answerinfect)
{
    for (my $new_infection = 0; !$new_infection;)
    {
        my $contact = int(rand($pop));

        if ($pop[$contact]{status} == WELL)
        {
            $pop[$contact]{status} = SICK;
            $new_infection         = 1;
        }
    }        
}

print Dumper(\@pop);
printf "You have infected %d %s: ", $answerinfect,
                                   ($answerinfect == 1) ? 'person' : 'people';
print join(', ', sort map { $_->{name} } grep { $_->{status} == SICK } @pop);