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#!/opt/perl/bin/perl
use strict;
...
    $mutex->unlock;
}

#!/opt/perl/bin/perl
use strict;
...
close $fh;
print "\n";

#!/opt/perl/bin/perl
use strict;
...
close $fh;
print "\n";

  time perl script_sem.pl   | wc -l     # 0.543s OP's example
  time perl script_mutex.pl | wc -l     # 0.663s Ditto, MCE::Mutex
  time perl script_mce.pl   | wc -l     # 0.225s MCE, no chunking
  time perl script_chunk.pl | wc -l     # 0.077s MCE, chunking enabled

  time perl script_sem.pl   | wc -l     # 1.072s OP's example
  time perl script_mutex.pl | wc -l     # 0.726s Ditto, MCE::Mutex
  time perl script_mce.pl   | wc -l     # 0.255s MCE, no chunking
  time perl script_chunk.pl | wc -l     # 0.112s MCE, chunking enabled

  time perl script_sem.pl   | wc -l     # 1.849s OP's example
  time perl script_mutex.pl | wc -l     # 0.803s Ditto, MCE::Mutex
  time perl script_mce.pl   | wc -l     # 0.339s MCE, no chunking
  time perl script_chunk.pl | wc -l     # 0.179s MCE, chunking enabled