Others have pointed to reasons why you get the error message. Let me try to guess what you want instead:

use strict; use warnings;

my %value_for;
$value_for{'row10'} = 1;
#
# later...
my $key = 'row' . 10;
print $value_for{$key};
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If you want a hash, you have to give it a name, unless you make it an anonymus hash of course.