In your line:

foreach $sedolelement (keys $tradedatehash{$tradedatemarker}) {
[download]

the element $tradedatehash{$tradedatemarker} is going to be a hash reference, not a hash directly. This is because perl stores complex structures as references as opposed to direct storage (see perlref for more details). To convert this hashref to a hash variable that keys can use, you can 'dereference' it by surrounding the variable with %{ }, as in:

foreach $sedolelement ( keys %{ $tradedatehash{ $tradedatemarker } } )
+ { ... }
[download]

(Similarly, array references are deref'ed with @{ }, if you need this)

-----------------------------------------------------
Dr. Michael K. Neylon - mneylon-pm@masemware.com || "You've left the lens cap of your mind on again, Pinky" - The Brain
"I can see my house from here!"
It's not what you know, but knowing how to find it if you don't know that's important