Variables whose names begin with control characters are forced into main:: no matter what package you are in. This is how things like ${^TAINT} work (which is a variable named "\ctAINT" -- note that "\ct" is CTRL-T). This sounds like a simple bug where "control character" has been implemented as something like "not ' '..'~'" or "not /^[a-z_]/i".

Note that this bug does not require 'use utf8' as this code:

use strict;
my $ì= 10;
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just uses plain 8-bit Latin1 and results in: Can't use global $^= in "my", near "my $ì" which also hints that I'm correct about the source of the bug since it reports the variable name as "$^=".

Update: Ah, a different bug with "unusual" variable names.

        - tye