use CGI or die;

Really. The $INPUT{'users'} indicates you're using something else, which is a might bad idea for many reasons.

my @delete_users = param('users');

my %user = map { /^([^\|]*)\|(.*)/ }, <FILE>;
close FILE;

delete %user{@delete_users};

open (FILE, ">$datafile") or die "Can't write: $!";

while(my ($usr,$pwd) = each %user) {
    print FILE "$usr|$pwd\n";
    print "$pwd<br>";
}
close(FILE);
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Makeshifts last the longest.