The difference is that I didn't use it as an lvalue. Perl checks whether or not it can be used as an lvalue before trying to execute it. 
>perl -e"&vec()"
Undefined subroutine &main::vec called at -e line 1.

>perl -e"&vec() = 1"
Can't modify non-lvalue subroutine call at -e line 1.

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In reply to Re^9: What would you change? by ikegami
in thread What would you change? by BrowserUk

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