#!/usr/bin/perl

use strict;
use warnings;

use File::Find;
use File::Copy;

#
# Set this to 0 to turn off printing which files it's moving.
#
my $verbose = 1;

my @dirs = qw(
    Folder_1
    Folder_2
    Folder_3
    Folder_4
);

my %num;

sub wanted {

    if ( -f $_ ) {

        my $dir = $File::Find::dir;

        $num{$dir}++;

        my $src = "$dir/$_";
        my $target = "${dir}_$num{$dir}.jpg";
        my $mv_msg = "move '$src' to '$dir/$target'";

        if ( -f $target ) {

            die "Can't $mv_msg: File already exists!\n"

        }
        else {

            move( $_, $target ) || die "Can't $mv_msg: $!\n";

            print "$mv_msg OK\n" if ($verbose);

        }

    }

}

File::Find::find( { wanted => \&wanted }, @dirs );