If the string representation of your integers is longer than their packed representation, then you will save a few bytes per key, but the savings will be minimal. A few hundred kilobytes at most:

undef $h{ $_ } for 1e6 .. 2e6;;
print total_size( \%h );;
34617643

undef $i{ pack 'V', $_ } for 1e6 .. 2e6;;
print total_size( \%i );;
34231634

undef $j{ $_ } for 100e6 .. 101e6;;
print total_size( \%j );;
34883144

undef $k{ pack 'V', $_ } for 100e6 .. 101e6;;
print total_size( \%k );;
34239854
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And the relative saving is less still if you are assigning values to the keys.

If you described your application and the nature/distribution of your keys in more detail, there are often ways of storing such data that has significantly lower memory costs. Though these usually trade space for speed.