In Perl 5, the .. operator in scalar context is a "flip flop" which is false until its left operand of it evaluates as true, and then is true in subsequent checks until the right operand is true (and then potentially true and false over and over again).

So you were checking something like "As long as $interval is 10 and until it is 60", not that it is between 10 and 60. If you read about this useful operator in perlop you'll see that as a special case, when an operand is just a number, that's taken as a line number in the input. Either way, this doesn't have much to do with what you were attempting!

Incidentally, Perl 6 allows you to write:

die $usage unless $interval ~~ 10 .. 60;         # note use of "smart 
+match" ~~ operator
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