At the top:

use Fcntl qw(:flock);
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Immediately after the 'read open':

flock(FH, LOCK_SH);
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Change the 'write open' to:

open(FH, "<+ $gb") or die "Could not open $gb: $!\n";
flock(FH, LOCK_EX);
seek(FH,0,0);
truncate(FH,0) or die "Could not truncate $gb: $!\n";
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Once you use flock, make sure that every process that accesess the files also uses flock.

Also, the "local $/" should be in it's own block, otherwise anything else you add to the bottom of this script will be in "ultraslurp mode"