gistfile1.pl

use strict;
use File::Copy;

foreach my $file (map { glob ($_) } @ARGV)
{
    # get a filename for the backup file
    my $backup = $file . ".bak";
    $backup = $` . ".bak" if ($file =~ /\.\w+$/);
 
    # copy the file to the backup
    copy ($file, $backup) or die "Can't copy $file to $backup: $!\n";
 
    # open them both, reading from BACKUP and writing to FILE
    open (BACKUP, "<", $backup) or die "Can't open $backup: $!\n";
    open (FILE, ">", $file) or die "Can't open $file: $!\n";
 
    # loop on backup and process each line
    while (<BACKUP>)
    {
        # insert your s/// code here!!
        s/this/that/g;
        print FILE $_;
    }
    close FILE;
    close BACKUP;
}