johngg@shiraz:~/perl/Monks > perl -Mstrict -Mwarnings -E '
my @files = qw{
   981139804_ABCDEF12345678765463565E_28062016172508.TXT
   981139804_ABCDEF12345678765463565E_28062016192508.TXT
   981139804_ABCDEF12345678765463565E_28062016112508.TXT
   981139804_ABCDEF12345678765463565E_28062016102508.TXT
   981139804_ABCDEF12345678765463565E_28062017102508.TXT
   971305332_XXXXXX12345678765463565E_28062011102508.TXT
   971305332_AAAAAAAA12345678765463565E_28062011102508.TXT
   971305332_CCCC12345678765463565E_28062011102508.TXT
   971305332_CCCC12345678765463565E_28062020102508.TXT
   };

my @oldest = do {
   my %seen;
   grep { not $seen{ ( split m{_} )[ 0 ] } ++ }
   map  { substr $_, 21 }
   sort
   map  {
            my( $col1, $dt ) = ( split m{[_.]} )[ 0, 2 ];
            my( $d, $m, $y, $t ) = unpack q{a2 a2 a4 a6}, $dt;
            pack q{a9 a4 a2 a2 a6 a*}, $col1, $y, $m, $d, $t, $_;
        }
   @files;
   };

say for @oldest;'
08971305332_AAAAAAAA12345678765463565E_28062011102508.TXT
08981139804_ABCDEF12345678765463565E_28062016102508.TXT