The simplest solution I found to the OP issue was to divide the track list in two halves, then play each one of them alternately:

## pseudocode
my @list = shuffle get_tracks;
my @list1 = @list[0..@list/2-1];
my @list2 = @list[@list/2..$#list];
while (1) {
    play @list1;
    play @list2;
    shuffle @list1;
    shuffle @list2;
}
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Regarding this:

Now, how you gonna make sure that NIN isn't followed by Elton John.

I have thought about a database with style tags and a mechanism for specifying which styles are allowable after a given song:

## pseudocode
my @style_levels = (
    [ qw/noise/               ],
    [ qw/metal heavy/         ],
    [ qw/pop rock/            ],
    [ qw/chill_out classical/ ],
);
sub get_next_song {
    my $cur_song = shift;
    my $cur_style = get_style_from_db $cur_song;  ## now we have 'pop'
+, 'classical'...
    my $index = get_index $cur_style;             ## walk @style_level
+s and return the index where 'pop' or 'classical' has been found
    my $next_index = $index + (-1,0,1)[rand 3];   ## randomly go to ne
+xt, current or previous style level (TODO: bounds checking)
    my $next_style = ${ $style_levels[$next_index] }[ rand @{$style_le
+vels[$next_index]} ];  ## choose a style of this level
    return get_song { style => $next_style };     ## pick some song of
+ this style
}
[download]