If i'm not mistaken, the explanation given on the Perl idioms explained page is the same as the explanation i offered in my previous comment which I quite here 
are you creating a reference to this variable $gene->display_id and th
+en deferencing it straight away with ${ }

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I'm double checking as I'm not very confident with perl

In reply to Re^6: Variable interpolation of object methods by Anonymous Monk
in thread Variable interpolation of object methods by Anonymous Monk

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