#!/usr/bin/perl

use strict;

# suppose original files are in directory "source", and
# we want to put edited versions into directory "edited":

my @files = <source/*>;  # that's called a "file glob"

for my $ifile ( @files ) {
    my $ofile = $ifile;
    $ofile =~ s/source/edited/;
    open( I, "<", $ifile );
    open( O, ">", $ofile ) or die "$ofile: $!";
    while (<I>) {
        s/foo/bar/g;
        print O;
    }
    close I;
    close O;
}