If I were you, I'd also standardize the naming format and lowercase everything while I was at it, but maybe that's just me.

use strict;
use warnings;

my @files = qw|
filename=merilyn14.jpg.jpeg
filename=003_Merilyn23.jpg.jpeg
filename=890FoxyM.jpg.jpeg
filename=006.jpg.jpeg
|;

my ($file, $folder);
for (@files) {
    next if !s/^filename=//;
    s/(\.jpe?g)+$//;
    $file = "$_.jpg";
    s/^\d+_?//;
    s/_?\d+$//;
    $folder = $_ || 'FileNum';
    print "$file -> $folder\n";
}

__DATA__
OUTPUT:

merilyn14.jpg -> merilyn
003_Merilyn23.jpg -> Merilyn
890FoxyM.jpg -> FoxyM
006.jpg -> FileNum
[download]