#!/usr/bin/perl
use warnings;
use strict;
use File::Find;

my $base_dir = '...';  # put in your base directory

find( \&wanted, $base_dir );

sub wanted {
    return if $_ eq '.' or $_ eq '..';
    if (-d) {
        print " >>> dive into: $_\n" if -d;
    }
    else {
        readout_file($_);  ## call subroutine readout_file
    }
}

sub readout_file {
    my ($filename) = @_;
    open my $fh, '<', $filename or die "can't open file:$!";
    while (<$fh>) {
        chomp;
        s/ /,/;        ## OR s/ /,/g; if you want 
        print $_, $/;  ## OR any other subroutine you want 
    }
 }