Ah -- sorry, I didn't catch the issue about passing the pipe file handle to XML::Parser.

If the idea of using IO::Unread doesn't work out (though I expect it will do fine), there's also the possibility that you can use a script like the one I suggested, but instead of having two different subs in the dispatch table, you would have two different command lines -- one being your (separate) XML parser script invoked in a way to handle one type of stream, and the other(s) for handling the other type(s) of stream, and you just launch the right one as a subprocess:

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 );
        }
    }
}
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Your separate parser script uses the command line option to control how to set up event handlers for parsing.

(updated snippet to fix stupid mistakes, like forgetting to use a list for "command", "option" when opening the pipe to the parser script.)