Woah.

The easiest way is to use an anonymous sub:

     $i    = 3;
     $func = sub { cos ($_[0]) };

     print "Das ist ",&$func($i),"\n";
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"cos" isn't a defined subroutine, it's a Perl thing, and AFAIK, you can't reference it as "\&cos" like you can for your own subs.

The following code doesn't work because of the aforementioned reason:

     $i    = 3;
     $func = "cos";

     print "Das ist ",&{$func}($i),"\n";
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This produces "Undefined subroutine &main::cos called at evl line 7."