Try this:

#!/usr/bin/perl 
use strict;
use warnings;

$^I = '~';

while (<>) {
    s/\s+myOldPass/ Onl33In05h15notmyreallpass/g;
    s/\s+myDBUser/ my-secret-db-user/g;
    print;
}
[download]

See perldoc perlvar for description of the $^I variable. Alternatively you can use -i switch for perl.

Update: note also that you don't need foreach to loop through @ARGV. <> will loop automatically.

Update2: fixed extension