Actually, Perl knows when you've used double quotes around a fixed string and changes them to single quotes internally.

$ perl -MO=Deparse -le 'print "Fixed string"'
BEGIN { $/ = "\n"; $\ = "\n"; }
print 'Fixed string';
-e syntax OK
$
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In my opinion, a much better reason for using single quotes for fixed strings is that it tells the next programmer to look at your code that nothing interesting is going on in that string.