C:\>type convert.pl
#!/usr/bin/perl -w -i.bak
use strict;
while (<>) {
    s/this/that/g;
    print;
}

C:\>type this.txt
this
this
this
this

C:\>perl convert.pl this.txt

C:\>type this.txt
that
that
that
that

C:\>type this.txt.bak
this
this
this
this

C:\>