Please describe the nature of the problem you are experiencing. You've given us code (or is it pseudo-code?), but you haven't actually told us what problems you're seeing with it. Is it not compiling? Is a lookup for that username later failing to give you the result you're expecting? If the responses above don't hint at the problem, consider providing us with a little more information.

Some simple debugging could also go a long way here:

use Data::Dumper;

print "\$columns[0] = '$columns[0]'\n";
$users{$columns[0]} = 'x';   # or whatever
print Dumper(\%columns);     # give us a nice view of %columns
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