I think choroba’s point is that you can use BEGIN blocks to determine when the warning is issued. For example:

#! perl
use strict;
use warnings;

BEGIN      { print "BEGIN 1\n"; }
sub each   { warn  'main::each() was called' }
BEGIN      { print "BEGIN 2\n"; }
sub delete { warn  'main::delete() was called' }
BEGIN      { print "BEGIN 3\n"; }

my %person = (name => 'Ken Takakura');

while (my ($key, $val) = each %person)
{
    print "$key: $val\n";
}
    
delete $person{name};

BEGIN      { print "BEGIN 4\n"; }
[download]

produces this output:

BEGIN 1
BEGIN 2
BEGIN 3
Ambiguous call resolved as CORE::each(), qualify as such or use & at 1
+66_SoPW.pl line 13.
BEGIN 4
name: Ken Takakura
[download]

which shows that the warning is issued during the compile phase (before the main code is executed) at the point where the code specifies an ambiguous call to each.

This works because BEGIN blocks are executed during compilation, as soon as they are seen by the Perl compiler, before the main code is run. See BEGIN, UNITCHECK, CHECK, INIT and END.

HTH,