Can anyone explain me what sets my expections out of sync with reality?
Probably because can isn't what you think it is. Try UNIVERSAL::can()

use DBI;
my $dbh=DBI->connect("dbi:SQLite:dbname=dbfile");
warn $dbh;
print "can error: ", $dbh->can("error"),"\n";
print "UNIVERSAL::can((", UNIVERSAL::can($dbh,'error'),"\n";
eval {print $dbh->error('eeep');};warn $@ if $@;
bless $dbh, "My::Special::DBI::db";
warn $dbh;
print "can error: ", $dbh->can("error"),"\n";
print "UNIVERSAL::can((", UNIVERSAL::can($dbh,'error'),"\n";
eval {print $dbh->error('eeep');};warn $@ if $@;
print "\n-----\n\n";

package My::Special::DBI::db;
use base qw(DBI::db);

sub error {
   my $dbh=shift;
   return $dbh->errstr();
}
__END__
DBI::db=HASH(0x19b7f1c) at dbi.dbd.error.subclass.pl line 3.
can error:
UNIVERSAL::can((
Can't locate object method "error" via package "DBI::db" at dbi.dbd.er
+ror.subclass.pl line 6.
My::Special::DBI::db=HASH(0x19b7f1c) at dbi.dbd.error.subclass.pl line
+ 8.
can error:
UNIVERSAL::can((CODE(0x19978e4)

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