s### is the substitution operator using the # character as the delimiter instead of the more conventional s/// delimiters. The $ denotes the end of the string (perlre).

Tip #6 from the Basic debugging checklist: B::Deparse can help identify this.

Update: correction thanks to johngg

In reply to Re: can understand one line of code by toolic
in thread can understand one line of code by anaconda_wly

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