DB<100> @option{qw/a b c/}=()
 => (undef, undef, undef)
 
  DB<101> exists $option{a}
 => 1                         #true
 
  DB<102> exists $option{d}
 => ""                        #false
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but smart-match should be safe as long as you don't forget the quotes.

Nice hash slice, thanks!

Just let me polish up your code so other newbies don't get confused:

my %option;
@option{qw/a b c/}=();

# my %option = (
#  'a' => undef,
#  'b' => undef,
#  'c' => undef
# );

exists $option{a} and print 'true';

exists $option{d} or print 'false';
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PS: what the h**l is DB <...> ?

DB <...> : the code was run under the debugger. Example of a debugger session:

$ perl -de 42

Loading DB routines from perl5db.pl version 1.32
Editor support available.

Enter h or `h h' for help, or `man perldebug' for more help.

main::(-e:1):   42
  DB<1> %hash = qw( a 1 b 2 c 3 d 4 );

  DB<2> print %hash
c3a1b2d4
  DB<3> print scalar %hash
4/8
  DB<4> $hash{e} = undef

  DB<5> print scalar %hash
5/8
  DB<6> print "true" if defined $hash{e}

  DB<7> print "true" if exists $hash{e}
true
  DB<8>
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