have a look at use diagnostics to improve error messages (and read them ;-)

use strict;
use warnings;
use diagnostics;


for $i (0..10) {
    print $i;
}
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read the part about lexically scoped variables:

Global symbol "$i" requires explicit package name at /tmp/tst2.pl line
+ 6.
Global symbol "$i" requires explicit package name at /tmp/tst2.pl line
+ 7.
Execution of /tmp/tst2.pl aborted due to compilation errors (#1)
    (F) You've said "use strict" or "use strict vars", which indicates
+ 
    that all variables must either be lexically scoped (using "my" or 
+"state"), 
    declared beforehand using "our", or explicitly qualified to say 
    which package the global variable is in (using "::").
    
Uncaught exception from user code:
    Global symbol "$i" requires explicit package name at /tmp/tst2.pl 
+line 6.
Global symbol "$i" requires explicit package name at /tmp/tst2.pl line
+ 7.
Execution of /tmp/tst2.pl aborted due to compilation errors.
 at /tmp/tst2.pl line 10
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if you don't know what lexically scoped is, have a look at Coping with Scoping

PS: crossposting provokes downvotes! :)