The difference between " and ' in Perl

Question

According to this comment:

You also should not use single quotes in print ">>${ '_<$filename' }<<\n". Instead try: print ">>${ \"_<$filename\" }<<\n"

I always thought that differences between " and ' are only that the string is interpreted or not.

I want to ask why at this context:

print ">>${ \"_<$filename\" }<<\n"
print ">>${  '_<$filename'  }<<\n"

perl prints different values?

Why I should use \" instead of just ' here?

Answer 1

What happens is that in both cases, $filename is not interpolated by the outer string (">>...<<\n"), but rather by ${ ... }. (this is just deduction from the behavior). This means that in ">>${ '_<$filename' }<<\n", $filename isn't interpolated at all, while it is in ">>${ \"_<$filename\" }<<\n".

If it was interpolated by the outer string, then you'd ran into some troubles if your string was containing quotes:

$filename = "ab'cd";

If interpolation was done by the outer string, then "${'$filename'}" would be equivalent to "${'ab'cd'}", which is a syntax error. Or maybe rather to "${ab'cd}", which is equivalent to "${ab::cd}"; not what you want either.

Whereas if the interpolation is done by ${}, then in "${'$filename'}", ${...} is really ${"$filename"} (without the escaped double-quotes), which interpolates $filename, and you get something like ${"ab'cd"}, like you'd want.

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Consider this example:

$abcd = "First var";
${'$v'} = "Second var";
$v = "abcd";

say "${\"$v\"}"; # prints "First var"
say "${'$v'}";   # prints "Second var"

It shows that with "${\"$v\"}", $v was interpolated, whereas with "${'$v'}", it wasn't.