Either you want inplace edit with the $^I variable; or write the results to a temporary file created with File::Temp::tempfile and then rename the new file to the original one.

Here's an example for the second one:

use warnings; 
use strict; 
use File::Temp "tempfile"; 
my $i = "yourfile.txt"; # filename
my ($O, $o) = tempfile; 
open my $I, $i or die "open: $!"; 
while (<$I>) { 
    s/foo/bar/g; 
    print $O $_; 
} 
close $O or die;
rename $o, $i;
__END__
[download]