In Perl, what's the best way to turn this string: 

7705554200-4299

into

7705554200-7705554299

I know how to grab the first 6 digits but not sure how to insert them after the hyphen.

```
#!/usr/bin/perl

use strict;
use warnings;

my $num = '7705554200-4299';

# Check to see number is a range
if ($num =~ /-/)
{
  # Pull area code and exchange
  my $extract = substr($num, 0, 6);
  print "Area code + exchange: $extract\n";
}

```