use strict; 
use warnings; 
use 5.012; 
 
my %results; 
 
while ( my $line = <DATA> ) { 
    chomp $line; 
 
    my @pieces = split ' ', $line;  #splits on any group of whitespace 
    my $key = $pieces[3]; 
 
    push @{ $results{$key} }, $line; 
     
} 
 
use Data::Dumper; 
say Dumper \%results; 
 
 
for my $key (sort keys %results) { 
    my @lines = @{ $results{$key} }; 
 
    for (sort @lines) { 
        say; 
    } 
    say "*" x 20; 
} 
 
 
__DATA__ 
201211  b    c     2 a 
201203    y z 1 x 
201201   bb cc 2 aa 
201210 yy zz 1    xx

--output:-- 
$VAR1 = { 
          '1' => [ 
                   '201203    y z 1 x', 
                   '201210 yy zz 1    xx' 
                 ], 
          '2' => [ 
                   '201211  b    c     2 a', 
                   '201201   bb cc 2 aa' 
                 ] 
        }; 
 
 
201203    y z 1 x 
201210 yy zz 1    xx 
******************** 
201201   bb cc 2 aa 
201211  b    c     2 a 
********************

use strict; 
use warnings; 
use 5.012; 
 
my %results; 
 
while ( my $line = <DATA> ) { 
    chomp $line; 
 
    my @pieces = split ' ', $line; 
    my $key = $pieces[3]; 
 
    push @{ $results{$key} }, \@pieces; 
     
} 
 
use Data::Dumper; 
say Dumper \%results; 
 
for my $key (sort keys %results) { 
    my $AoA = $results{$key}; 
    my @ordered_refs = sort { @$a[0] cmp @$b[0] } @$AoA; 
 
    for my $ref (@ordered_refs) { 
        say "@$ref"; 
    } 
    say "*" x 20; 
} 
 
__DATA__ 
201211  b    c     2 a 
201203    y z 1 x 
201201   bb cc 2 aa 
201210 yy zz 1    xx

--output:-- 
$VAR1 = { 
          '1' => [ 
                   [ 
                     '201203', 
                     'y', 
                     'z', 
                     '1', 
                     'x' 
                   ], 
                   [ 
                     '201210', 
                     'yy', 
                     'zz', 
                     '1', 
                     'xx' 
                   ] 
                 ], 
          '2' => [ 
                   [ 
                     '201211', 
                     'b', 
                     'c', 
                     '2', 
                     'a' 
                   ], 
                   [ 
                     '201201', 
                     'bb', 
                     'cc', 
                     '2', 
                     'aa' 
                   ] 
                 ] 
        }; 
 
201203 y z 1 x 
201210 yy zz 1 xx 
******************** 
201201 bb cc 2 aa 
201211 b c 2 a 
********************