use Proc::Background;

my $proc = Proc::Background->new(
    "perl -MFcntl=:flock -e 'open FH, \">>\", \"$filename\";exit flock FH, LOCK_E
X|LOCK_NB",
);

my $rc = $proc->wait >> 8;
ok( !$rc, "Cannot flock $logfile" );