There is no runtime difference unless there is a variable being interpolated, in which case only double quotes are appropriate. The perl parser treats them the same otherwise: 
perl -MO=Deparse -e ' $a = "foo"; $b = ''foo''; $c = q{foo}'  
$a = 'foo';
$b = 'foo';
$c = 'foo';

[download]
perl -MO=Deparse -e ' $a = "$foo"; $b = q{$foo};'  
$a = "$foo";
$b = '$foo';

[download]

In reply to Re^3: Use of single quotes versus double quotes by imp
in thread Use of single quotes versus double quotes by lokiloki

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