I may be wrong, but I don't think you are supposed to have those brackets around your offset. Another way to do what you want is simply to do something like...

my($user, $var) = split /:/, $_, 2;
$foo{$user} = $var;
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Or maybe I've misunderstood something

$_.=($=+(6<<1));print(chr(my$a=$_));$^H=$_+$_;$_=$^H;
print chr($_-39); # Easy but its ok.
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