I see the SOPW has a better answer already... but I'm posting this anyway in order to illustrate a process other than the add print statements suggested above.

Inspection of Email::Sender::Simple shows sub header { starting at Ln 176:

sub header {              # Ln 176
  my ($self, $field) = @_;

  my $headers  = $self->{headers};
  my $lc_field = lc $field;
                             # Ln 181
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Based on what you've told us, one or both the variables that are supposed to be passed to the sub isn'/aren't getting there. Could that be something that will be supplied by main::id?

Alternately, I can only suggest that you track thru the module's source to find the call and backtrack from there to find why data isn't getting to $field