This behavior is naively unexpected by me; can somebody please explain this mechanism?

You're expecting too much from a debugging feature of CGI.pm

The CGI protocol communicates through %ENV , and each child process gets the same %ENV as parent process, so to CGI.pm won't use the debugging freature and look at @ARGV

But since CGI programs don't populate @ARGV you can use

#!/usr/bin/perl  --
use strict;
use warnings;

use CGI ();

Main( @ARGV );
exit( 0 );

sub Main {
    my $cgi = @_ ? CGI->new(@_) : CGI->new;
}
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Or it might be prudent to use GetOpt::Long / --query=a=1;b=2;c=3... instead of testing only for @ARGV