Trap 2 - a solution 

• Assume $_ is volatile

while(my $line = <$fh>) {
  print $line if contains_badger($line);
}

• Or for the more paranoid

while(defined(my $line = <$fh>)) {
  print $line if contains_badger($line);
}

• But don't do this

while(my($line) = <$fh>) {
  print $line if contains_badger($line);
}