my @border = ( [2,1], [1,4], [4,5], [5,2], [2,1] );
my $area = 0;
for ( 0 .. $#border-1 ) {
    $area += ( $border[$_+1][0] - $border[$_][0] )
           * ( $border[$_][1] + $border[$_+1][1] ) / 2;
}
print "$area\n";