#!/usr/bin/perl -w
use strict;
my %User;

{
    local @ARGV;    
    @ARGV = qw(/etc/passwd /usr/defined/file);    
    while (<>) {
        chomp;
        my @field = split /:/;
        unless (exists $User{$field[0]}) {
            $User{$field[0]} = \@field;
        }
        else {
            print "WARNING:  Duplicate found for $field[0]\n";
        }
    }
}
my $testuser = "blah";
if (exists $User{$testuser}) {
    print "$testuser exists on system\n";
    print "Field 4 for $testuser is $User{$testuser}->[3]\n";
}