perl $_ 及 $'

open open_file, "/etc/passwd";
while (<open_file>) {
   if ($_ =~ /^root:/) {
      print "$'";
   } elsif ($_ =~ /^bin/) {
      last;
   }
}
close open_file;

顯示

x:0:0:root:/root:/bin/bash

當 $_ 時

open open_file, "/etc/passwd";
while (<open_file>) {
   if ($_ =~ /^root:/) {
      print "$_";
   } elsif ($_ =~ /^bin/) {
      last;
   }
}
close open_file;

顯示

root:x:0:0:root:/root:/bin/bash

也就是使用 $' 會直接去除我過濾的再顯示出來