一段DBM文件的操作

dbmopen (%HASH, "dbmfile", 0666) or
    die "文件打不开!";
print $HASH{'John'} if (exists $HASH{'Hohn'});
$HASH{'Mary'} = '0227331122';

@sort_keys = sort {$a cmp $b} (keys %HASH);
for (@sort_keys) {
    print "$_: $HASH{$_}\n";
}

delete $HASH{'Paul'};
dbmclose(%HASH);
程序没看明白谁能帮看下这程序是在干吗
(exists $HASH{'Hohn'})这句是什么意思？