Lawliet's method (copying the hash) is the only way I can think of meeting your requirements. However, if you'll settle for keeping the key while having its value undefined, you can avoid copying the hash.

my %hash = ('key1' => 'v1', 'key2' => 'v2');

del_hash_key(\%hash);

sub del_hash_key {
    my $hash = shift;
    local $hash->{'key1'} = undef;  # Until end of scope.
    print %$hash; # Prints the latter key/value pair
    print "\n";
}

print %hash; # Prints both key/value pairs
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