$ perl -le'my %x = "A" .. "F"; print "@{[ %x ]}"; $_ = lc for %x; prin
+t "@{[ %x ]}";'
C D A B E F
C d A b E f
[download]

So, no, you can't modify hash keys directly.

However, you can do this:

$ perl -le'my %x = "A" .. "F"; print "@{[ %x ]}"; $x{ lc() } = delete 
+$x{ $_ } for keys %x; print "@{[ %x ]}";'
E F A B C D
c D a B e F
[download]