The odd-looking syntax

mysubroutine {block} @list;
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is mentioned in perlsub. Look in the section on prototypes where it describes the "&" prototype character, particularly the sentence "An & requires an anonymous subroutine, which, if passed as the first argument, does not require the sub keyword or a subsequent comma" and also the examples "try", "catch", and "mygrep".
 
On the other hand, it looks to me that map is somewhat more magical than the examples in perlsub; if you call

map (sub { "1" x $_ }, (1..10));
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it returns an array of coderefs (references to subroutines) which is not at all what is needed here.
 
Also, not all builtin functions have regular enough syntax to be expressed with a prototype at all. map is one of them:

perl -wle 'print prototype("CORE::map")'
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prints

Use of uninitialized value in print at -e line 1.
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