Doing it that way both if and unless will fail. If you don't declare the sub up front you need to call the sub with parens: check_disk_space(). There is some weirdness though. With if the error is

Bareword "check_disk_space" not allowed while "strict subs" in use...
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and with unless

Bareword "check_disk_space" not allowed while "strict subs" in use...
Bareword "check_disk_space" not allowed while "strict subs" in use...
[download]

Not sure why it comes up twice.

I prefer to have all my subs at the end of the script and always call them with the parens. I use parens with perl built ins too if the args have any level of complexity. YMMV.