If you want to keep the regex based approach try the code below, and see 'perldoc perlre' for the meaning of /m and /s modifiers. (However I think BrowserUK's solution above is more lightweigth than this.) 
#! /usr/bin/perl

use strict;
use warnings;

my $config = do { local $/; <DATA> };
my @interfaces = ( $config =~ m/^interface.+?\n!\n/msgi );
foreach my $interface (@interfaces) {
  print $interface, "--\n";
}

__DATA__
interface GigabitEthernet6/10
 description TRUNK-KBV-SW001
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan 1,30,31,45,156-158
 switchport mode trunk
 switchport nonegotiate
!
interface GigabitEthernet6/11
 description Offices
 switchport access vlan 31
 spanning-tree portfast
!
interface GigabitEthernet6/12
!

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In reply to Re: Matching a cisco interface config by rubasov
in thread Matching a cisco interface config by FloydATC

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