This difference between single and double quotes has nothing to do with your using CGI, it is much more basic to the language than that. Double quotes interpolate, single do not (other than translation of \\ to \, and \' to '). Try it:

my $foo = "This is a variable";
print "Double quotes: ($foo)\n";
print 'Single quotes: ($foo)\n';
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A book recommendation. merlyn's book Learning Perl gives an excellent overview of Perl that should do a lot to clear up any confusion about how quoting works, the various kinds of data structures, writing functions, what regular expressions are, and so on. I think it would be very helpful to you at your current stage of Perl development.