The only thing that can be passed to a sub is a list of scalars. You did not pass a hash to the sub, and the new hash you created in the sub isn't an IxHash. Solutions:

sub printh {
    tie my %x, 'Tie::IxHash', @_;  # Recreate the hash. Expensive
    warn $_ for keys %x;
}

printh(%h);  # Flatten the hash
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sub printh {
    my ($x) = @_;
    warn $_ for keys %$x;
}

printh(\%h);  # Pass a reference
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