G'day bowei_99,

Your text is made up of a series of elements each consisting of a hash (#) followed by a variable number of characters which aren't hashes, i.e. "[#][^#]+".

The "last chunk", as you called it, doesn't need to be part of the replacement. Just match the first "#...", a pair of "#...", the "#..." you want to move; then move the 3rd capture before the 2nd:

$ perl -Mstrict -Mwarnings -le '
    my $text = <<LIST;
#EXTM3U
#EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=80000
80.m3u8
#EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=400000
400.m3u8
#EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=700000
700.m3u8
#EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=1500000
1500.m3u8
#EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=2500000
2500.m3u8
LIST
    $text =~ s{\A([#][^#]+)((?:[#][^#]+){2})([#][^#]+)}{$1$3$2}m;
    print $text;
'
#EXTM3U
#EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=700000
700.m3u8
#EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=80000
80.m3u8
#EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=400000
400.m3u8
#EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=1500000
1500.m3u8
#EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=2500000
2500.m3u8
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