Please see haukex's detailed explanation here. Briefly,  T::c (as a "bareword" literal, without any quotes) is not a "simple identifier"; it will not automatically be stringized to use as a hash key. On the other hand, anything that already is a hash key is a string; hash keys are always and can only be strings. Because hash keys are always strings, data serialization utilities like Data::Dumper will put quotes around them. (How a particular hash key got to be a string in the first place is another story.)

c:\@Work\Perl\monks>perl -wMstrict -MData::Dumper -le
"my %my_hash;
 $my_hash{T::c} = 'xxx';
 print $my_hash{'T::c'};
 print Dumper \%my_hash;
"
Bareword "T::c" not allowed while "strict subs" in use at -e line 1.
Execution of -e aborted due to compilation errors.

c:\@Work\Perl\monks>perl -wMstrict -MData::Dumper -le
"my %my_hash;
 $my_hash{'T::c'} = 'xxx';
 print $my_hash{'T::c'};
 print Dumper \%my_hash;
"
xxx
$VAR1 = {
          'T::c' => 'xxx'
        };
[download]