fcntl will lock appropriately across NFS:

#!/usr/bin/perl -w

use strict;
use Fcntl qw(F_WRLCK SEEK_SET F_SETLKW);

open(F,"+< $ARGV[0]")
  or die "Couldn't lock myself: $!\n";
my $args = pack("sslli",F_WRLCK,SEEK_SET,0,0,$$);
fcntl(F,F_SETLKW,$args)
  or die "Couldn't lock: $!\n";

print "File locked\n";
sleep(1000);
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note that the arguments to fcntl (in the pack statement) are system-specific.

IIRC, compiling Perl with its own flock (instead of using the OS-provided one) will use an implementation built on top of fcntl, which should be NFS-safe, simpler to use, and more portable.