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