The canonical form of a subroutine invocation is:

$result = &  { something_that_returns_coderef } ( @args );
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The braces around the coderef expression can be dropped only when the coderef is in a simple scalar variable. Yours was not. So you'd have to use:

& { $functions{$func} }();
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However, as of fairly recent Perls, you can also use an arrow form:

$functions{$func}->();
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This was added by Chip in 5.4 under some urging by me based on a bet I made that I couldn't get a new feature into 5.4 during the gamma golden release phase. I won. {grin}

-- Randal L. Schwartz, Perl hacker

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update: for more details, check out my Linux magazine column on subroutine references, and an older column from UnixReview.