It's too late now, but if perl had added a special syntax of m!! to indicate a negated match we could use m!\|!. As it is we have a modified form of one of sfink's suggestions: $_ !~ m/\|/.
Also, what about?
pipe(STDIN, STDOUT); #Optional: $|--, print STDOUT "This is not a pipe\n" ne <STDIN>;
I'm not belgian but I play one on TV. On dit que je parle comme un belge aussi.