Until a new reserved word is added to the language. I could see dec used as a decrement operation equivilent to $foo = $bar - 1;. Or maybe you have a module that imports an apr() sub into your namespace for calculating Annual Precentage Rates. Or maybe something completely different.
The point is, you can't guarentee that any of those won't take a different meaning at some point in the future, so you should just avoid using them as barewords.
"There is no shame in being self-taught, only in not trying to learn in the first place." -- Atrus, Myst: The Book of D'ni.