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use strict;
use warnings;
...
my( @sample ) = @entity{ 8, 8, 3, 1, 5, 0, 0 };

my $prob = probability( \@entity, \@sample );

sub probability {
    my $entity_count = @{$_[0]};
    my $sample_count = @{$_[1]};
    return( ( 1 / $entity_count ) ^^ $sample_count );
}