#!/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 );
[download]

Thanks serf, very useful, I optimized (for SEO purposes) an images gallery of about 4000 images. Thanks and Happy New Year!

File::Wildcard is designed for this kind of job. Warning: untested

use File::Wildcard;

my $wc = File::Wildcard->new(
        path => './//',
        match => qr{^(.*?Folder_\d+)/(\w+.jpg)$},
        derive => [ '$1_$2' ],
        );
while (my $match = $wc->next) {
        my ($from,$to) = @$match;
        print "Renaming $from to $to\n";
        rename $from $to;
}
[download]

Start with File::Find

perl -e 'while(<./*>){$folder=$_;$y=0;while(<$_/*.jpg>){rename $_, $fo
+lder."/".$folder."_".$y++.".jpg"}}'
[download]