grep {-x "$_/$program"} split /:/ => $ENV {PATH};
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And if $program starts with a slash, you can just check -x $program.

The other problem is more general. If the program exists, the fork and the exec will work. The other program *will* get started. The only "problem" is that it will terminate sooner than you expect. Hence, your pipe will be "broken". In that case (printing to a pipe that was closed on the other hand), your program will get a SIGPIPE. You could write something like:

    eval {
        local $SIG {PIPE} = sub {die "Pipe!"};
        print FH "Whatever\n";
        ....
        close FH or die;
    };
    if ($@ && $@ =~ /^Pipe!/) {
        ... Do something ...
    }
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Note that you might reach the end of the eval before the SIGPIPE is delivered though.

Abigail