Here's a multiple-language program, or polyglot, that prints out the well-known phrase. It can be run in Perl, C (with one compiler warning), Tcl, and REBOL.

#include <stdlib.h>
#include <stdio.h>
#define p \
<<END
set // [if 0 {
;
int main() {
  /* }}];
  puts "Hello world from Tcl!"
  if 0 { { */
  printf("Hello world from C!\n");
  return 0;
}
/*
END
;
print("Hello world from Perl!\n");
<<quit
}
exit
REBOL []
print "Hello world from REBOL!"
quit
;#*/
[download]

edit: removed an unnecessary #define, a fragment from an earlier version of the code. It was actually a somewhat cunning way to obfuscate the code, but this code isn't strictly designed just to frustrate the reader.

another edit: The "fix" broke it. It really does work now.