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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