Although I don't agree with the question, an interesting solution does exist, I guess... Don't have time to explain it, but it's not that hard to figure out.

#!/usr/bin/perl
use strict;
use warnings;
my @n = (2,3,4,7,8,9,10,8,20,9,7);
my $m;
foreach my $x (@n) {
  for (0..$#n) {
    my $y = $n[$_];
    my $diff = abs($x - $y);
    $m = $x if $_ == $#n;
    next if $diff == 0;
    last if $diff + $x == $y; # Then y > x.
  }
}
print $m;
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