use strict;
use warnings;
use 5.010;

use MIME::Lite;
use HTML::TreeBuilder; # or what you like most
use File::Basename;
use File::Type;

# ...

sub makemsg {
    my (%id, %saw);
    (my $tree=HTML::TreeBuilder->new)->parse_file(shift);
    for my $img ($tree->look_down(_tag => 'img')) {
        defined(my $imgname=$img->attr('src')) or 
          warn "[$0] Warning: img tag with no src attribute!" and next;
        unless($id{$imgname}) {
            # not necessary, but I prefer it like that!
            (my $id=basename $imgname) =~ s/_*[^\w]+/_/g;
            $id++ while exists $saw{$id};
            $saw{$id}++;
            $id{$imgname}=$id;
        }
        $img->attr(src => "cid:$id{$imgname}");
    }
    my $msg=MIME::Lite->new(
        To =>'someone@example.org',
        Subject =>'HTML with in-line images!',
        Type =>'multipart/related');
    $msg->attach(
        Type => 'text/html',
        Data => $tree->as_HTML);
    
    $tree->delete; # However ugly, it's necessary!
    
    my $ft=File::Type->new;
    for (keys %id) {
        $msg->attach(
            Type => $ft->checktype_filename($_) // 'image/xyz',
            Id   => $id{$_},
            Path => $_);
    }
    $msg;
}