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

...
$str=~s/$rex/$1-$2/g;      # and now substitute in

print "After  : $str\n";

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