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delete EXPR

Deletes the specified key(s) and their associated values from a hash. For each key, returns the deleted value associated with that key, or the undefined value if there was no such key. Deleting from $ENV{} modifies the environment. Deleting from a hash tied to a DBM file deletes the entry from the DBM file. (But deleting from a tie()d hash doesn't necessarily return anything.)

The following deletes all the values of a hash:

foreach $key (keys %HASH) {
    delete $HASH{$key};

And so does this:

delete @HASH{keys %HASH}

But both of these are slower than just assigning the empty list or undefining it:

%hash = ();         # completely empty %hash
undef %hash;        # forget %hash every existed

Note that the EXPR can be arbitrarily complicated as long as the final operation is a hash element lookup or hash slice:

delete $ref->[$x][$y]{$key};
delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};