use strict;
use warnings;

my @array = (1,5,6,7,8,55,63,77,89,103,104,105,106,200,215,554);

local $, = ', ';

print sort { $a <=> $b } 
    keys %{ { 
        map {
            ( $array[ $_ ] == $array[ $_ + 1 ] - 1 )
                ? ( $array[ $_ ] => 0, $array[ $_ + 1 ] => 0 )
                : ()
        } 0 .. $#array - 1
    } };