my %groupMap =
(
  'ABC' => 'groupA.txt',
  'DEF' => 'groupB.txt',
  'GHI' => 'groupC.txt' # and so on and so on
)

##</code><code>##


foreach my $n (keys %emp)
{
    my ($pattern,$file);
    while (($pattern,$file) = each %groupMap)
    {
        if ( $emp{$n}{'Org'} =~ /$pattern/ )
        {
            saveEmpToFile($emp{$n}{'Emp'}, $file);
        }
    }
}

sub saveEmpToFile
{
    my $emp = shift;
    my $file = shift;

    # note: openning for append
    open(OUTFILE, ">>$file");
    print OUTFILE $emp;
    close OUTFILE;
}