On the contrary, the warning does occur under perl 5.6.1 on Linux, as well as perl 5.6.0 and perl 5.7.1. Are you sure you had warnings turned on for all your tests?

The warning doesn't occur in 5.005_03 because the warning wasn't implemented until 5.6.0. :)

Anyway, this warning is actually produced at the end of the compile phase, after the entire script has been compiled:

warn "About to call\n";
mysub(1);
warn "Called\n";

BEGIN {
  package Foo;
  require Exporter;
  @ISA = qw(Exporter);
  @EXPORT = qw(mysub);
  
  sub mysub ($) { warn "mysub: @_\n" };
}
BEGIN {
  warn("About to import\n");
  Foo->import();
  warn("Imported\n");
}

CHECK { warn "Done compiling.\n" }
INIT { warn "Running.\n" }

__END__
About to import
Imported
main::mysub() called too early to check prototype at tmp.pl line 4.
Done compiling.
Running.
About to call
mysub: 1
Called
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