my @items = split /\s+/, $buf;
if (scalar(@items) == 13) {
    # process (ii)
}
else {
    # process (i) or (iii)
}

##</code><code>##

use strict;
use warnings;

while (<DATA>) {
    my @items = split /\s+/, $_;
    if (scalar(@items) == 13) {
         my (@new)  = ($items[4] =~ /(PV)(.*)/);
         my (@new2) = ($items[8] =~ /(RL)(.*)/);
         splice @items, 4, 1, @new;
         splice @items, 9, 1, @new2;
    }
    print "@items\n";   # now @items always contains same number of tokens
    # process items...
}

__DATA__
SS  21   PL 2#3  PV  51.3 CL #110 +0 RL 126' SA 106 DS 93
SS  21   PL 2#3  PVa51.3 CT^ 110 +0 RL126, SA 106 DS 93
SS  21   PL 2#3  PV   51.3 CL #110 +0 RL 126' SA 106 DS 93

##</code><code>##

SS 21 PL 2#3 PV 51.3 CL #110 +0 RL 126' SA 106 DS 93
SS 21 PL 2#3 PV a51.3 CT^ 110 +0 RL 126, SA 106 DS 93
SS 21 PL 2#3 PV 51.3 CL #110 +0 RL 126' SA 106 DS 93