open (FILE,"<$filename") or die "Cannot open: $!";
map{s/stuff/other_stuff/g;print}(<FILE>);

##</code><code>##

open (FILE,"<$filename") or die "Cannot open: $!";
@ary = map{s/stuff/other_stuff/g}(<FILE>);

##</code><code>##

perl -pe 's/stuff/other_stuff/g' filename.txt