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;
}
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