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use strict;
my $persistentVar = $defaultvalue;
...
  close FH 
    or warn "Failed to close $persistentStateFile, $!" and exit(1);
}

END {
    open FH, "< $0" or warn "Failed to preserve state... couldn't open
+ '$0', $!" and exit(1);
...
    print FH $_;
    close $file;
}