#! perl -slw
use strict;

$/ = "!\n";

while( <DATA> ) {
    next unless m[^interface];
    print "'$_'";
}

__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
!
stuff
with more stuff
!
interface GigabitEthernet6/11
 description Offices
 switchport access vlan 31
 spanning-tree portfast
!
thing
with other things
!
interface GigabitEthernet6/12
!

##</code><code>##

C:\test>junk
'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
!
'