my $filename = "input.txt";
open my $file, '<', $filename;
@fileinput=<$file>;
close($file);

foreach $line(@fileinput) { my $Cword=($line); if($Cword=s/(^AU:\s[a-z]\s\[a-z])/(^AU:\s[A-Z]\s[A-Z])/g) { print $Cword; }