"A workmate asked me the best way to ensure that only one copy of his Perl script is running at a time".
I just developed a perl script that needed to ensure exactly that since it is CGI and can be run my multiple users over the internet and I need to ensure only one conucrrent usage. I simply created a file called "status.txt" ont he server, and had the script overwrite its status on the file contents at start and exit - 0 was disabled, 1 was enabled but not in use, 2 was enabled and in use. I'm sure its not the quickest or best way, but as always with Perl, TMTOWDI and all that, and its simple and works just fine.
It also has the benefit of letting you examine the file contents yourself and deciding on action based on them. You could also use this to set a limit of say 2 concurrent access, or 3 or 4 or... you get the point, by simply incrementing on each script start and decrementing (is that a word?) on each script exit.
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