multiplexing (input stream can be converted into several output streams)

If you're thinking in terms of unixy file i/o streams, you probably can't do better than a good modern shell, like ksh or bash. They let you open several (not sure what the limit is) streams for reading and writing very easily. See, for example, ksh93 i/o or bash i/o.

If by "multiplexing" you mean that a single input stream would be (selectively?) directed to some number of output file handles or separate sub-processes, I don't see a need for anything beyond the standard "open()" call. Open as many output file handles as you need, whether they refer to disk files or to processes (using open my $fh, '|-', "process", @args). (Update: this sounds to me like it might be "de-multiplexing" (demuxing for short), which is sort of the opposite of "multiplexing" (muxing).)

As for syntactic simplicity, you are either going to be doing a simple loop over file handles for each input record (print to each output in turn), or else you'll put the file handles into a "dispatch table" hash, and print to the ones you want according to whatever decision needs to be made on a record-by-record basis.

#!/usr/bin/perl
use strict;
use warnings;

my $i = 0;
my @out_files;
foreach ( qw( op0 op1 op2 op3 ) ) {
    unless ( open $out_files[ $i ], '>', $_ ) {
        die "Cannot open $_ for writing: $!\n";
    }
    $i++;
}

while ( <> ) {
    foreach my $out ( @out_files ) {
        print $out $_;
    }
}
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#!/usr/bin/perl
use strict;
use warnings;

my $line;
my %out = map { $_ => undef } qw( op0 op1 op2 op3 );
( open $out{ $_ }, '>', $_ or die "can't: $!\n" ) for keys %out;
while ( $line = <> ) { print $_ $line for values %out; }
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#!/usr/bin/perl
use strict; use warnings;

my @out = qw( op0 op1 op2 op3 );
my %out = map { $_ => undef } @out;
for ( @out ) { open $out{ $_ }, '>', $_ || die "Can't: $!\n" }
while ( my $l = <> ) { print $_ $l for values %out }
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Thanks for all the help. I think what I'm looking for hasn't been done. I'll post it as soon as it's semi-usable. I do have a followup question: Does XSLT necessarily produce XML? Or can it spit out arbitrary data (for my case character data is sufficient)?

Yay the Monastery!