Actually, ref doesn't give the underlying implementation. In the example code you gave, it should give 'BLAH', which doesn't tell you if 'BLAH' is an arrayref, or a hashref:

for my $i ( [], {} ) {
    bless $i, 'BLAH';
    printf( "%s\t%s\n", ref($i), $i )
}
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I typically use tye's answer, and go with UNIVERSAL::isa. (eg, if I'm dealing with structures passed in from a SOAP call, and I want to make sure the structure is right, even if the class name is wrong, before I start to manipulate it)