Special variable $^i for Perl

In text processing often encounter such a situation: there are a number of files need to modify the contents, but also want to keep the original file content. This time Perl provides a very convenient variable $^i, will need to save the backup file suffix after the variable, when we modify the contents of the file, Perl will: 1, first open the original file and save as "the original filename +$^i the suffix character stored" as the file name of the backup files. 2, modify the original document. For example, in the following procedure, I will change the ABC in all. Dat text to DFG and back up to. bak files
#!/usr/bin/perl-w
Use strict;
$^i= ". Bak";
while (<>) {s/abc/dfg/g; Print }