my $username = "usernamehere";
print "Duplicate!\n" if getpwnam($username);

##</code><code>##

my $username = "user1";

while (<DATA>) {
    chomp;
    my @info = split ",";
    foreach (@info) {
        print "Duplicate!\n" if /\A\Q$username\E\z/
    }
}
        
__DATA__
user1
user2
user3
user4
user5,pass5,email5,ip5
user5,pass5,email5,ip5
user5,pass5,email5,ip5
user5,pass5,email5,ip5
user5,pass5,email5,ip5