my %emp = (
	987654 => { Emp=>'A. Employee', Org=>'ABC', '...'=> },
	987653 => { Emp=>'A.N.Other',   Org=>'DEF', '...'=> },
    #    .....
);


##</code><code>##


my %OrgEmp;
push @{ $OrgEmp{$_}{Org}}, $emp{$_}{Emp} for keys %emp;


##</code><code>##


my $re_org = qr[ '(' . join('|', qw[ABC DEF ...]) . ')' ]o;
my %OrgEmp;
for my $empNo (keys %emp) {
	push @{ $OrgEmp{$1} }, $emp{$empNo}{Emp} if $emp{$empNo}{Org} =~ $re_org;
}


##</code><code>##


for my $org (keys %OrgEmp) {
    open  OUT, '>', 'Group' . $org or die $!;
    print OUT "$_\n" for @{ $OrgEmp{$org} };
    close OUT;
}


##</code><code>##


my %groups = map{
	open my $fh, '>', "Group$_" or die $!;
	$_ => $fh;
} qw[ABC DEF ...];

for my $empNo (keys %emp) {
	for my $grp (keys %groups) {
		#! Note the {} (bare block) around the filehandle are necessary.
		print { $groups{$grp} } $emp{$empNo}{Emp} if $emp{$empNo}{Org} =~ m[$grp];
	}
}