You're exiting the loop too soon. You can't know if the guest isn't found until you've gone through the entire loop.

my $found;

open IPFILE, "< $ipfile" or die "couldn't read $ipfile: $!";
while (<IPFILE>) {
  last if $found = /\Q$remote_addr\E/o;
}
close IPFILE;

if ($found) { print "user found!\n" }
else { print "user not found!\n" }
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japhy -- Perl and Regex Hacker