The "my" obviously should be scrapped. demerphq went into that. $a and $b must be package/global variables. That's what sort counts on.

And for a more immediate answer to your question, check out these FAQ entries:

• How do I sort an array by (anything)?
• How do I sort a hash (optionally by value instead of key)?

For some more examples, check out perldoc on sort.

The gist of it all is this: the sort block is called to compare two items from the list-to-sort. It will actually be called many times for this one sort statement, in order to establish a complete sorting order for the whole list. The value returned by the block reflects whether the first item ($a) is to be considered bigger or smaller than the second item ($b), depending on the sign of the result. If they are equal, return 0. Your code block should return the same sign as cmp and <=> return, which is not a coincidence, but by design. You might as well use - (minus) for comparing numbers, the results would be quite the same.

print join " ", sort { $a - $b } 1, 3, 2, 5, 4;
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This code snippet simply numerically sorts the passed numbers, resulting in a rather boring list.