sub md5files {
    my $dir      = shift;           # where the files are (must be a 
                                    # direct path from root /)
    my $what     = shift || '*';    # what files, default = '*'
    my $md5file  = shift || "$dir/checksums.md5"; # default.
    my %hash_md5;
    
    my @t = split(/ /, $what);
   
    open (my $md5_outfh, '>', $md5file ) or die "Err: $!";
    
    foreach my $f ( @t ) {
        foreach my $file2md5 ( 
            grep { -f and $_ ne $md5file } <$dir/$f> 
        ) {
            next unless my $md5 = md5calc( $file2md5 );
            my ( $filename ) = $file2md5 =~ m{/([^/]+)$};
            say $md5_outfh "$md5\t$filename";
            $hash_md5{$md5} = $filename;  
        }
    }
    close $md5_outfh;
    return %hash_md5 or die $!; # Consider 'use autodie;'
}