I just wanted to follow up and explain _why_ your code doesn't work the way you expect and why it is necessary to use the notation shown by Tortue, Ovid, and lachoy above. 
push @$somehash{$foo.$bar}, $x;

# is the same as
push @{$somehash}{$foo.$bar}, $x;

[download]
Perl sees $somehash as a scalar holding a reference to a hash. Paraphrasing from the Panther, when parsing an expression like this, key lookups are done last. Since the hashref is dereferenced before the key lookups are done, the @ is applied to return a hash slice.

(aside: With a regular hash %somehash, a one-element hash slice (which is usually not what you want) would look like @somehash{$foo}, but with a scalar $somehash as a hashref, you would use @$somehash{$foo} or @{$somehash}{$foo} as above.)

But you didn't want a hash slice-- you wanted to dereference an array reference. In order to do that, you put braces around the scalar holding the array ref. Now we have @{ $somehash{$foo.$bar} }, which is the notation shown by the other monks earlier in this thread. The outer braces force the hash lookup to be done before the @ is applied to dereference the returned value (which, of course, is an array ref). I hope this helps explain what was happening in your code. Also, for a good reference on, well, references, check tye's excellent references quick reference (Re: push).

--sacked

In reply to (sacked: hash slice notation) Re: push onto hash! by sacked
in thread push onto hash! by Anonymous Monk

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