Perl interprets

{("foo","bar")}
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as a code block containing two constants, as you can see by the warnigns Perl gives:

perl -we "{('foo','bar')}"
Useless use of a constant in void context at -e line 1.
Useless use of a constant in void context at -e line 1.
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You need to tell Perl that you mean a hash constructor instead of a code block by prepending (for example) a + to the expression:

perl -we "+{(foo => 'bar')}"
Useless use of single ref constructor in void context at -e line 1.
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If you put that together, it works:

perl -we "print +{(foo => 'bar')}->{foo}"
bar
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