my $str="1 2 3 10 6 7 8 11 12 13 14 15 161";

print "Before : $str\n";

# we use qr to avoid the use re 'eval';
my $rex=qr/(\d+)                      # match a number put it in $1
           (?{$num=$1})               # set $num to be $1
           (?:                        # dont capture all of this,
             \s+                      #  must have space between the numbers
             (                        #  capture into buffer $2
               (??{'\b'.++$num.'\b'}) #   match only the successor of $num
             )                        #  end of buffer
           )+                         # one or more
         /x;
$str=~s/$rex/$1-$2/g;      # and now substitute in

print "After  : $str\n";

##</code><code>##

Before : 1 2 3 10 6 7 8 11 12 13 14 15 161
After  : 1-3 10 6-8 11-15 161