The syntax %{ ... } is expecting a hash ref, but you're handing it a list. You'll need to enclose your list in (another) set of curly brackets to get it to do what you want:

#     anon hash bracket \
#                        |
#    grouping bracket \  |
#                      | |
#                      V V
print "$_\n" for keys %{ { returnlist() } };
sub returnlist { 1..10 }
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-Blake