Pseudo-hashes are depreciated in perl 5.8 but they are still supported so "Pseudo hashes are depreceated" is only a warning. On the other hand, pseudohashes are completely unsupported in perl 5.10, so there you'd indeed get an error with the message "Not a HASH reference". Just try this standalone test:

perl -we 'warn ${[{"a",1},"foo"]}{"a"}'
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In perl 5.8 you get a warnings but the right result:

Pseudo-hashes are deprecated at -e line 1.
foo at -e line 1.
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whereas perl 5.10 dies.