perl -e 'keys %h = 42'
# no errors

perl -e 'keys (%h = 42)'
# Type of arg 1 to keys must be hash (not list assignment) at -e line 1, at end of line
# Execution of -e aborted due to compilation errors.

perl -e '(keys %h) = 42'
# Can't modify keys in list assignment at -e line 1, at EOF
# Execution of -e aborted due to compilation errors.