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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