Maybe the line number in the error message is wrong. That happens (particularly when the error is in an elsif). If defined($data{'FLOATER'}) is true as you say, it makes no sense for the if to give you that warning.

Update: Additionally, you're probably better off passing the hash by reference. It saves memory, executes faster and preserves tie magic. For example:

sub test {
   my ($data) = @_;
   ...
   print($data->{'FLOATER'});  # Need to add "->"
   ...
}

my %data = ...;
test(\%data);
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