A hash key may or may not exist. (Update: If a hash key does not exist, its value is not defined.) If it exists, its value may or may not be defined. If its value is defined, it may or may not be true. Here are some ways to test if a key exists and its value is true. The first test (test A) is not sufficient.

c:\@Work\Perl\monks>perl -wMstrict -le
"my $hr = { foo => undef };
 ;;
 warn qq{test A: 'foo' exists, undefined \n};
 if (exists $hr->{foo}) {
   $hr->{foo} =~ /bar/;
   }
 ;;
 warn qq{test B: 'foo' exists, undefined \n};
 if (exists $hr->{foo} && defined $hr->{foo}) {
   $hr->{foo} =~ /bar/;
   }
 ;;
 warn qq{test C: 'foo' exists, undefined \n};
 if ($hr->{foo}) {
   $hr->{foo} =~ /bar/;
   }
 ;;
 $hr = {};
 warn qq{test D: 'foo' does NOT exist \n};
 if ($hr->{foo}) {
   $hr->{foo} =~ /bar/;
   }
"
test A: 'foo' exists, undefined
Use of uninitialized value in pattern match (m//) at -e line 1.
test B: 'foo' exists, undefined
test C: 'foo' exists, undefined
test D: 'foo' does NOT exist
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Update: See True or False? A Quick Reference Guide; Truth and Falsehood in perlsyn.