After perusing through the perldsc, you might come up with something like the following code (untested):

my $johndoe_hours = 0;
foreach my $month ( keys %main } {
   $johndoe_hours += $johndoe_hours + $main{$month}{johndoe};
}
[download]

my $johndoe_hours = 0;
foreach my $month ( keys %main } {
   $johndoe_hours += $main{$month}{johndoe};
}
[download]

If you want to get rid of the error message when {johndoe} does not exist in the hash $main{$month} (that is assuming you are using warnings), you might want to do something like the following instead:

my $johndoe_hours = 0;
foreach my $month ( keys %main } {
   if ( exists $main{$month}{johndoe} ) {
   $johndoe_hours += $johndoe_hours + $main{$month}{johndoe};
   }
}
[download]

my $johndoe_hours = 0;
foreach my $month ( keys %main } {
   if ( exists $main{$month}{johndoe} ) {
   $johndoe_hours += $main{$month}{johndoe};
   }
}
[download]

update: Fixed code from doubling johndoe's hours.

-enlil