send_email (  {
       subject => "Test subject",
       body    => "Test body",
  } );

sub send_email {

   my $param = shift;

   my $subject     = $param->{subject};
   my $body        = $param->{body};
   my $is_dev_mode = exists $param->{is_dev_mode}
     ? $param->{is_dev_mode}
     : undef;

   $is_dev_mode = 1
       unless ( $is_dev_mode == 0 || $is_dev_mode == 1 )
       ; # In case the CGI param is ''; default is to be "safe" (is_dev_mode == 1)

   _blast_email_to_all_my_important_customers( $subject, $body ) unless ( $is_dev_mode );
}

##</code><code>##

use strict;
use warnings;

send_email (  {
       subject => "Test subject",
       body    => "Test body",
  } );

sub send_email {

   my $param = shift;

   my $subject     = $param->{subject};
   my $body        = $param->{body};
   my $is_dev_mode = exists $param->{is_dev_mode}
     ? $param->{is_dev_mode}
     : undef;

   $is_dev_mode = 1
       unless ( $is_dev_mode == 0 || $is_dev_mode == 1 )
       ; # In case the CGI param is ''; default is to be "safe" (is_dev_mode == 1)

   warn "is_dev_mode is '", $is_dev_mode, "'\n";

   _blast_email_to_all_my_important_customers( $subject, $body ) unless ( $is_dev_mode );
}
##</code><code>##

Use of uninitialized value $is_dev_mode in numeric eq (==) at C:\Users\davel\Desktop\test-of-dev-mode-flag-revised.pl line 19.
Use of uninitialized value $is_dev_mode in warn at C:\Users\davel\Desktop\test-of-dev-mode-flag-revised.pl line 23.
is_dev_mode is ''