$cols contains a reference to a hash. %$cols (short for %{$cols}) means to access the hash referenced by the reference in $cols. See perlref.

my @sorted_col_names = sort { length($a) <=> length($b) || $a cmp $b }
                       keys %$cols;

foreach my $col_name (@sorted_col_names) {
   my $cell = $col->{$col_name};
   print("$cell\t");
}

print("\n");
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