my @holdem;
my %dispatch = (
    one => [ qw/my_parser --this_way/ ],
    two => [ qw/my_parser --that_way/ ], 
);
my $target;

open( $pipe, "-|", "some_xml_generator" );  # not "ls", obviously
while ( <$pipe> ) {
    if ( $target ) {
        print $target $_;
    }
    else {
        push @holdem, $_;
        my $decision = look_for_evidence( $_ );
        if ( $decision =~ /one|two/ ) {
            open( $target, "|-", @{$dispatch{$decision}} )
                or die "failed to launch: $dispatch{$decision}: $!\n";
            print $target $_ for ( @holdem );
        }
    }
}