Aristotle's answer to Switching on internal UTF-8 flaq on DBI result from database made me find the following phenomenon: 
$a = [[0]];
foreach (@$a) { $_++ foreach @$_ }
print $a->[0][0],$/;
$_++ for map @$_, @{$a};
print $a->[0][0],$/;
__END__
1
1

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The expected output is: 
1
2

[download]
The strange thing is that the extra list flattening with map() doesn't work. I don't see any reason why it shouldn't. I guess the question is: is this a bug or not?

Liz

Update:
Not a bug, just copying of the list with map(). Thanks to !1, blokhead and holo for the enlightenment.

In reply to $_ and list flattening with map() by liz

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