my %hash;
while (<DATA>) {
   my @data = split;
   $hash{$data[0]}->{employee_number} = $data[1];
   $hash{$data[0]}->{years_employed} = $data[2];
}
foreach (sort {
              $hash{$a}->{years_employed}
              <=>
              $hash{$b}->{years_employed}
              } keys %hash) {
   print "$_ $hash{$_}->{employee_number} $hash{$_}->{years_employed}\n";
};

__DATA__
SS123 EMP123 3
SS234 EMP124 2
SS456 EMP125 1

##</code><code>##

SS456 EMP125 1
SS234 EMP124 2
SS123 EMP123 3