#!/usr/bin/perl

use strict;

while (my $line = <DATA>) {
    print $line;
    if ( $line =~ /\sip address/ ) {
        $line =~ s/(\d+.\d+.\d+.)252/${1}100/;
        print $line;
    }
}

__DATA__
 ip vrf forwarding ABC:Defgh
 ip address 1.1.1.1 255.255.255.252
 ip vrf forwarding ABD: Efghi
 ip address 2.2.2.2 255.255.255.252