This is quick-n-dirty, and works just fine...

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
    } };
[download]

It does assume that @array is pre-sorted, and contains no duplicates. I was trying to come up with a regexp solution but gave up after confusing myself with (??{...}) constructs. ;) ...Enjoy.

Updated: Discarded unnecessary use of a named hash. ;)