aloha. bit late but i should not miss this opportunity to advertise ken williams' totally insane Tie::Text module;

here goes: following file = masssub.pl

use Tie::TextDir;
($old, $new, $dir) = @ARGV;
tie %dirs, 'Tie::TextDir', ($dir || "."), 'rw';
map { $dirs{$_} =~ s/$old/$new/g } keys %dirs;
untie %dirs;
[download]

invoke via massub.pl subthis forthis inthisdirectory

careful to specify the dir or it will sub in the working one.