OK, quick hack of your code gives me this:

#!/usr/bin/perl

use strict; 
use warnings;


my @test = ( 123, 12345, 12345678, "abc-abc", "abc-abcd" );

foreach my $match ( @test )
{
  if ( $match =~ m/\d{5,8}|\w{3}-\w{4}/ )
  {
    print "$match match\n";
  }
  else 
  {
    print "$match doesn't match\n";
  }
}
[download]

Which seems to work.
(Note that I have used \w instead of \D. See the manpage for what the difference is).
But like the man said in the previous post. perldoc perlre.

--
It's not pessimism if there is a worse option, it's not paranoia when they are and it's not cynicism when you're right.