The simple O(n) solution: 
use Test;

use strict;

INIT { plan tests => 5 }

sub issorted(&@) {
    my $code = shift;
    local $a, $b;
    for my $i (0..$#_-1) {
        ($a, $b) = @_[$i, $i+1];
        &{$code}() <= 0 || return 0;
    }
    1;
}

ok(issorted {$a cmp $b} ('a'..'z'));
ok(issorted {$b cmp $a} reverse ('a'..'z'));
ok(! issorted {$a <=> $b} (1..9,3,10));
ok(issorted {$a <=> $b} (1..10));
ok(issorted {$a cmp $b} (1,1,1,1,1));
ok(issorted {$a <=> $b} ());
ok(issorted {$a <=> $b} (1));
ok(issorted {$a <=> $b} (1,2));
ok(!issorted {$a <=> $b} (2,1));

[download]
Update 20070629: Added tests for low element count boundary condition. Already passes.

- Miller

In reply to Re: Fast way to check if a sort was doen by wind
in thread Fast way to check if a sort was doen by Outaspace

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