This page on Recursive Editing of files has a very nice example of a script (below) that does (I believe) just what you want. It also demonstrates some nice features such as skipping directories...

#!/usr/bin/perl

use File::Find;

@ARGV = ('.') unless @ARGV;

$dir = shift @ARGV;

find( \&edits, $dir );

sub edits() {

    return unless -f;    #skip directories

    $seen = 0;
    $file = $_;

    #Uncomment next line if you want multi-line edits
    #undef $/;

    local $^I = ".backup";

    #Warning - heavy magic here
    local @ARGV = ($file);

    while (<>) {

        #Remember to use the s option if doing multiline edits
        $seen++ if s/Lesson/Chapter/;

        print;

    }

    print "Found in $File::Find::name\n" if $seen > 0;

    #Comment out if you want to keep the backup
    #unlink $file.".backup";

}
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The title of your node (Read entire file from given folder) made me think that perhaps you wanted to read an entire file in one go, in which case you may want to take a look at File::Slurp.

HTH,