If you want to know whether it really, truely is a hash, see if SVt_PVHV is true for the SV. See sv.h for the dirt.

use B qw( svref_2object SVt_PVHV );
sub is_hash {
    my $ref = shift @_;
    return ref( $ref )
        and svref_2object( $ref ) & SVt_PVHV;
}
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If you just want to know whether it supports being treated like a hash, do something akin to a ->can test for hash-ness. If the hash dereference doesn't die, then $ref acts like a hash.

sub does_hash {
    my $ref = shift @_;
    return ref( $ref )
        and eval { %$ref; 1 };
}
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