Loop through the lines of your input file, search for your magic string with a regular expression, then inject the extra lines. Something like this should get you started: 
use strict;
use warnings;

while (<DATA>) {
    if (/ip helper-address/) {
        print;
        print " ip helper-address x.x.x.x\n";
        print " ip helper-address x.x.x.x\n";
    }
    else {
        print;
    }
}

__DATA__
!
interface GigabitEthernet1/0/1
 description *** VLAN 1 **
 no switchport
 ip address x.x.x.x x.x.x.x
 ip helper-address x.x.x.x
 no ip redirects
 no ip proxy-arp
 service-policy input U_SET_DSCP
 ip ospf cost 10
 srr-queue bandwidth share 1 3 97 1
 priority-queue out 
 standby ip x.x.x.x
 standby timers 1 3
 standby priority 95
 standby preempt
!

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In reply to Re: Appending data to a file in a specific location by toolic
in thread Appending data to a file in a specific location by nemasys

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