Although you show us how you generate the password list, which doesn't seem relevant at all, you don't provide us with sample failing user list data. Your substitution acts on $_ (the default variable) which takes the value of each entry in @nup in turn. If you see an undefined variable warning then most likely there is an undef in @nup nad that implies something bad about your users data.

Try providing a sample user data file with failing data in a __DATA__ section following your sample program. Something like:

use warnings;
use strict;

my @genpass = (1 .. 3);
my @nup = (<DATA>);

foreach (@nup) {
    s/xyz456/$genpass[0]/;
    shift (@genpass);
}

__DATA__
1
2
3
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Though it's certainly well intentioned and not unreasonable to draw attention to the content of the "user list" data file, I can't imagine how the OP code could be getting an "undef" element in the @nup array.

Have you ever encountered a situation where @array = <FILE> produces an undef element in the array? What sort of condition in a data file would cause that?

I think ig's question below is more likely to be on the mark.

Is it possible that when you try to substitute the passwords your ldif file has more than 53 lines? You are shifting one password off @genpass for each line in your ldif file. This is one password for each user only if your ldif file has one line for each user.

ig,
That worked the 2nd time that I tried it. I'm not sure why it didn't work the first time.
Thanks!

I would probably write your script something like the following, avoiding the problem of how many passwords to generate.

use strict;
use warnings;

my $users = "new.users";
open(my $in, '<', $users) or die "Cannot open $users: $!";

my $tmp = $users . '.tmp';
open(my $out, '>', $tmp) or die "Cannot open $tmp: $!";

foreach (<$in>) {
    if(/^userPassword: xyz456$/) {
        my $pw = generatePassword(10);
        s/^userPassword: xyz456/userPassword: $pw/;
    }
    print $out "$_";
}

close($out);
close($in);

unlink($users) or die "unlink($users): $!";
rename($tmp, $users) or die "rename($tmp, $users): $!";

exit(0);



sub generatePassword {
    my $length = shift;
    my $password = "";
    my $possible = 'abcdefghijkmnpqrstuvwxyz23456789ABCDEFGHJKLMNPQRST
+UVWXYZ';
    while (length($password) < $length) {
    $password .= substr($possible, (int(rand(length($possible)))), 1);
    }
    return $password
}
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It looks like you want something like this:

#!/usr/bin/perl
use warnings;
use strict;
use Tie::File;

my $users = '/home/v0x9849/scripts/new.users';

tie my @nup, 'Tie::File', $users or die "Cannot open $users: $!";

for ( @nup ) {
    if ( $_ eq 'xyz456' ) {
        $_ = generatePassword( 10 );
        }
    }

untie @nup;

sub generatePassword {
    my $length = shift;
    my @possible = ( 'a' .. 'k', 'm', 'n', 'p' .. 'z', 2 .. 9, 'A' .. 
+'H', 'J' .. 'N', 'P' .. 'Z' );
    return join '', map $possible[ rand @possible ], 1 .. $length;
    }
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open (USERS, ">", $users) or die "Cannot open $users: $!";
        foreach (@nup){
#       print "$genpass[0]\n";  <==this works
        s/xyz456/$genpass[0]/;  <==this gets uninitialized
        #print USERS;
        $a = shift(@genpass);
}
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open (USERS, ">", $users) or die "Cannot open $users: $!";
foreach (@nup){
    chomp;
    print "$genpass[0]\n";   
    s/xyz456/$genpass[0]/;   
    #print USERS;
    $a = shift(@genpass);
}
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I'd change it to something a bit more meaningful so not only that you'll remember what it does when you look at the code 6 months from now but you won't get messed up if you use the <=> operator in the program down the road.

s/xyz456/$genpass[0]/;
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means

$_ =~ s/xyz456/$genpass[0]/;
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You checked whether $genpass[0] was undef or not, but you didn't check $_.

#!/usr/bin/perl -w
use strict;
use warnings;

my @genpass;

my $pass = "new.pass";
open (PASS, ">", $pass) or die "Cannot open $pass: $!";

my $p = 1;
while ( $p < 53 ){
    @genpass = generatePassword(10);
    print PASS "@genpass \n";
    $p++;
}
close PASS;

open (PASS, $pass) or die "Cannot open $pass: $!";
@genpass = (<PASS>);
chomp @genpass;
print scalar (@genpass) , " <- genpass\n";

my $users = "new.users";
#open (USERS, "<", $users) or die "Cannot open $users: $!";
my @nup = (<DATA>);
print scalar (@nup) , " <- nup\n";
#close USERS;

open (USERS, ">", $users) or die "Cannot open $users: $!";
foreach my $line (@nup){
    $line =~ s/xyz456/$genpass[0]/;  
    print USERS $line;
    shift(@genpass);
}
 
close USERS;

sub generatePassword {
    my $length = shift;
    my $password = "";
    my $possible = 'abcdefghijkmnpqrstuvwxyz23456789ABCDEFGHJKLMNPQRST
+UVWXYZ';
    while (length($password) < $length) {
        $password .= substr($possible, (int(rand(length($possible)))),
+ 1);
    }
    return $password;
}

__DATA__
dn: uid=1234567,ou=people,dc=abc,o=oursite.com
objectClass: posixAccount
objectClass: shadowAccount
objectClass: account
objectClass: top
uid:  1234567
cn:  1234567
uidNumber:  31472
gidNumber:  7546
gecos:  First Last,ABCDEFG297,#######
homeDirectory: /home/1234567
loginShell:  /bin/csh
userPassword: xyz456   

dn: uid=1234567,ou=people,dc=abc,o=oursite.com
objectClass: posixAccount
objectClass: shadowAccount
objectClass: account
objectClass: top
uid:  1234567
cn:  1234567
uidNumber:  31472
gidNumber:  7546
gecos:  First Last,ABCDEFG297,#######
homeDirectory: /home/1234567
loginShell:  /bin/csh
userPassword: xyz456  

dn: uid=1234567,ou=people,dc=abc,o=oursite.com
objectClass: posixAccount
objectClass: shadowAccount
objectClass: account
objectClass: top
uid:  1234567
cn:  1234567
uidNumber:  31472
gidNumber:  7546
gecos:  First Last,ABCDEFG297,#######
homeDirectory: /home/1234567
loginShell:  /bin/csh
userPassword: xyz456
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