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open my $FH, ">>foo" or die "can't open";
flock $FH, LOCK_EX or die "no lock";
seek ($FH, 0, 2);

open my $LOCK, "<foo" or die "can't open for locking";
flock $LOCK, LOCK_EX or die "Can't lock";
...
print $FH "I own this file\n";
close $FH;
close $LOCK;