in reply to Modifying hash keys via aliasing

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 }

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 }

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