shift here shifts from the @ARGV array. If it fails, (if not enough file names, 3, were supplied on the command line), then the program dies, (quits), and prints the string contained in $usage, ("merge_bed.pl <input1> <input2> <output>". (This message tells the user that his perl program, merge_bed.pl or whatever name you choose as your program name requires 2 input filenames and 1 output name). | [reply]
[d/l]
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Can you explain the $usage and "shift or die" part?
If you employ Basic debugging checklist (deparse,print) you can figure it out pretty quick :)
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