Could we see some code? There are multiple things that could have gone wrong. If you have not already, please use strict; in your script and declare all variables with local scope.

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

del_hash_key(\%hash);

sub del_hash_key {
    my $params = shift;
    my %hash = %$params;
    delete $hash{'key1'};
    print %hash; # Prints the latter key/value pair
    print "\n";
}

print %hash; # Prints both key/value pairs
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