I'm passing a block to reduce.

Impossible. There's no such data type. You are passing a sub reference to reduce.

reduce BLOCK LIST
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is compiled as

&reduce(sub BLOCK, LIST)
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Fair enough. However, the sub is called with inherited @_ and it happens to work.

Same thing with map:

  sub foo { map { print $_[$_] } 0 .. $#_ }
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Can I not rely on this behavior?

Update. example code:

use List::Util 'reduce';
sub minindex { reduce { $_[$a] < $_[$b] ? $a : $b } 0 .. $#_ }

my @g = (55, 88, 33, 6, 234, 234, 52, 6, 1324, 22, 1234);

print minindex @g;              # $g[7] == 6
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This is perl 5, version 12, subversion 3 (v5.12.3) built for x86_64-linux-thread-multi
List::Util $VERSION = "1.22";

Update2. One final observation:
I also see coderef passed to the sub when run under debugger. But not when run normally.

Ikegami is right, thats not working!

Better pass the list as a closed over array (here @x)

  DB<109> @a=  map { int rand 100 } 1..10
 => (96, 10, 99, 9, 43, 8, 20, 85, 42, 26)

  DB<110> sub minindex { 
             my @x=@_; 
             reduce { $x[$a] < $x[$b] ? $a : $b } 0 .. $#_ 
          }

  DB<111> print minindex @a
5
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update

or even better like ikegami now suggested as ref to avoid overhead

  DB<114> @a=  map { int rand 100 } 1..10 => (22, 15, 27, 44, 70, 85, 
+74, 93, 96, 14)

  DB<115> sub minindex { 
             my $x=\@_; 
             reduce { $x->[$a] < $x->[$b] ? $a : $b } 0 .. $#_ 
          }

  DB<116> print minindex @a
9
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Cheers Rolf

( addicted to the Perl Programming Language)

Don't copy @_, take a reference to it.