In the first case () is in scalar context so is a pair of empty brackets, not the empty list you may have been expecting. There is no error because => is really just a comma that implictly quotes the bare word to the left. The anon hash is thus the equivelent of {'foo', }. (See List value constructors and Comma Operator.)

Why you get different behaviour in the debugger I don't know however.

Update: strike bogus scalar context comment - see reply.

DWIM is Perl's answer to Gödel

In reply to Re: warnings under the debugger by GrandFather
in thread warnings under the debugger by trwww

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