Based on the results I'd say because keys cannot be lvalues 
$ perl -MData::Dump -e "sub f{dd\@_; $_++ for @_;} my %g = 1..4; dd\%g
+; f(%g); dd\%g; "
{ 1 => 2, 3 => 4 }
[1 .. 4]
{ 1 => 3, 3 => 5 }

$ perl -MData::Printer -e "sub f{p@_; $_++ for @_;} my %g = 1..4; p%g;
+ f(%g); p%g; "
{
    1   2,
    3   4
}
[
    [0] 1,
    [1] 2,
    [2] 3,
    [3] 4
]
{
    1   3,
    3   5
}

[download]

So yeah, that seems to be the cause 

$ perl -MData::Dump -e " my %g = 1..4; dd\%g; $_++ for keys %g; dd\%g;
+ $_++ for values %g; dd\%g; "
{ 1 => 2, 3 => 4 }
{ 1 => 2, 3 => 4 }
{ 1 => 3, 3 => 5 }

[download]

I'm sure its documented somewhere, I leave that up to you :)

In reply to Re: Modifying hash keys via aliasing by Anonymous Monk
in thread Modifying hash keys via aliasing by vsespb

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