Though I'm responding to Zaxo, my comment applies to most of the answers I see. With code doing a split, check that $pair[1] aka $ip is actually getting set to a defined value. Otherwise, your hash value may be undef on bad input but you won't get a warning about it until you actually try to use the hash value later on. Best to validate as you go:

while (<$fh>) {
   chomp;
   my ($host, $ip) = split;
   warn("bad hosts line: $_"),next if !defined $ip;
   $hosts{$host} = $ip;
}
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Note that if you check defined($ip) the chomp is necessary, since otherwise $ip set to the empty string (taken from the empty string following the newline character). If you use a regex to split up the line instead, make sure it actually requires both fields and check if the match succeeds (which gets a little funky if you are using map):

%hosts = map {
   if (/^(\w+)\s(.+)/) {
      ($1 => $2)
   } else {
      (warn "bad hosts line: $_")[1..0];
   }
} <FILE>;
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