Here's the trick:
the keys function returns an array.
When you do

keys @import_values{"ship","weight","dwg","rev"}
[download]

You are causing something very funky

%import_values{"ship","weight","dwg","rev"}
[download]

Is a syntax error.

@import_values{"ship","weight","dwg","rev"}
[download]

Is trying to take a hash slice, which works, but returns an array, and keys expects a hash.

As others have said, just use keys %hash.

The other (not as good, but preserves order of keys) way would be:

foreach $value3 (@import{'ship','weight','dwg','rev'})
[download]

See perldata, line 867 or so, for discussion of hash slices