# untested with your particular situation, but should be a guide.
 eval {
        local $SIG{ALRM} = sub { die "lock\n" }; # NB: \n required
        alarm 5;
        flock(LOCK, LOCK_EX | LOCK_NB);
        alarm 0;
    };
    if ($@) {
        die unless $@ eq "lock\n";   
    } else {
        # keep going
    }