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use strict;
use warnings;
...
   }
   map  { [$_, $frequencies{$_}] }
   keys %frequencies;

ItemID    1995    1996    1997    1998    1999    Freq.
20        =       =       =       =       =       5    
...
19                =                       =       2    
15                                =               1    
18                                =               1