I'm sure I've done this previously, but how escapes me just now: 

sub testit{ print 'hi' };;

sub doit (&) { $_[0]->() };;

$codeRef = \&testit;;

doit( $codeRef );;
Type of arg 1 to main::doit must be block or sub {} (not scalar derefe
+rence)

[download]

Thanks.

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In reply to The & prototype and code references in scalars. by BrowserUk

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