stevieb's approach is probably the way to go, however it assumes that if BR_IP = "n" then you go and search CoreVlanIP for matches. In the code below I assume that if BR_IP = "n" then do nothing, but if the @brnwlist is empty then search CoreVlanIP for matches. I have added some print statements and use Data::Dumper to show what is going on (a habit worth acquiring): 
use Data::Dumper;
my $count = 1;

foreach my $server (@{$servers->{server}}) {
    print "\nCount: ", $count++, "\n";
    my $brlan = $server->{BRLAN} ;
    print "brlan = $brlan\n";
    next unless $brlan eq "y";
    
    my $brip = $server->{BR_IP};
    my $num = $server->{CoreLanNum};
    my $corebrip = $server->{'CoreLanIP_'.$num.'__1'} ;
    my $brmask  = $server->{'CoreLanNetmask_'.$num};
    my @brnwlist;
    for my $n (1..$num){
        $corebrip = $server->{'CoreLanIP_'.$n.'__1'} ;
        if (not $corebrip) {
            print "CoreLanNum $n - No value for CoreLanIP_".$n."__1\n"
+;
            next;
        }
        print "CoreLanNum $n - brip: $brip, corebrip: $corebrip\n";
        $brmask  = $server->{'CoreLanNetmask_'.$n};
        if ( $corebrip eq $brip ){
            my $brlist = join ("/", $brip, $brmask) ;
            $brlist =~  s/.[0-9]*\//.0\//g;
            push( @brnwlist, $brlist) unless $seen{$brlist}++;
        }
        else {
            print "coreip does not match brip\n";
        }
    }
    print "brnwlist: ", Dumper(\@brnwlist);
    next if @brnwlist;
    
    my @vnwlist;
    my $vnum = $server->{CoreVlanNum};
    for my $i (1..$vnum){
        my $vlanip = $server->{'CoreVlan_IP_'.$i};
        if (not $vlanip) {
            print "CoreVlanNum $i - No value for CoreVlan_IP_$i\n";
            next;
        }
        print "CoreVlanNum $i - brip: $brip, vlanip: $vlanip\n";
        my $vmask  = $server->{'CoreVlan_Netmask_'.$i};
        if ($vlanip eq $brip){
            my $vlist = join ("/", $brip, $vmask) ;
            $vlist =~  s/.[0-9]*\//.0\//g;
            push(my @vnwlist, $vlist) if ! $seen{$vlist}++;
        }
        else {
            print "vlanip does not match brip\n";
        }
    }
    print "vnwlist: ", Dumper(\@vnwlist);
}

[download]
Output: 
Count: 1
brlan = n

Count: 2
brlan = y
CoreLanNum 1 - brip: 192.168.165.16, corebrip: 192.168.156.16
coreip does not match brip
CoreLanNum 2 - brip: 192.168.165.16, corebrip: 192.168.156.16
coreip does not match brip
brnwlist: $VAR1 = [];
CoreVlanNum 1 - brip: 192.168.165.16, vlanip: 192.168.156.16
vlanip does not match brip
CoreVlanNum 2 - No value for CoreVlan_IP_2
vnwlist: $VAR1 = [];

Count: 3
brlan = y
CoreLanNum 1 - brip: 192.169.73.1, corebrip: 192.169.32.1
coreip does not match brip
CoreLanNum 2 - brip: 192.169.73.1, corebrip: 192.169.32.1
coreip does not match brip
CoreLanNum 3 - brip: 192.169.73.1, corebrip: 192.169.73.1
CoreLanNum 4 - brip: 192.169.73.1, corebrip: 192.169.73.1
brnwlist: $VAR1 = [
          '192.169.73.0/255.255.255.240'
        ];

[download]

In reply to Re: Use of uninitialized value $brip in join or string at script.pl line 50 by tangent
in thread Use of uninitialized value $brip in join or string at script.pl line 50 by deep27ak

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