pipethread is a module by rkeene that implements thread-based concurrency and parallelism. It gives you the | from sh in Tcl, for Tcl code.
proc a {inchan outchan} { puts $outchan blah }
pipethread::pipe a | { gets $inchan line; puts $outchan "[string length $line]:$line" }
set output [pipethread::pipe exec ls | foreach line { puts $outchan "-->$line<--" }]
pipethread::pipe -outchan stdout -- exec ls | foreach line { puts $outchan "-->$line<--" }