Shouldn't the q{} operator return whatever is inside it without interpolation?

Yes. The only special sequences in single quotes are: backslash-backslash, backslash-delimiter and delimiter.

Wouldn't that be "$temp"?

Yes

$ perl -le'print q{"$temp"}'
"$temp"
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And when "$temp" is eval()'ed wouldn't the qq{} operator around $temp return a string with the $variables interpolated?

Yes. The value of $temp is interpolated.

$ perl -le'$temp=q{Bread was $1.50}; print eval q{"$temp"}'
Bread was $1.50

$ perl -le'$temp=q{Bread was $1.50}; print "$temp"'
Bread was $1.50
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This seems much easier; no sprintf necessary:

I don't know why you used sprintf in any of the examples. You do realize that

'...'
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is just as much an expression as

sprintf('%s', '...')
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right? An expression is a piece of code that can be used as an operand. That includes simple values such as a string.

I thought I followed the instructions in the docs to the letter, but eval EXPR just spits out errors.

Not true. eval EXPR doesn't spit out errors. It places them in $@ and you didn't even check $@. The message you posted came from say $y;.

What you're missing is that Perl programs are compiled and executed in their own scope, usually called "file scope". Think of the eval code as being inside a do{}. Yeah, that could be made more explicit.

$ perl -E'use strict; my $x = 10; eval q{ my $y = $x; }; say $y'
Global symbol "$y" requires explicit package name at -e line 1.
Execution of -e aborted due to compilation errors.

$ perl -E'use strict; my $x = 10; eval q{ my $y = $x; say $y };'
10

$ perl -E'use strict; my $x = 10; my $y; eval q{ $y = $x; }; say $y'
10

$ perl -E'use strict; my $x = 10; my $y = eval q{ $x }; say $y'
10
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eval {my $y = $x};

Don't try to apply the docs for eval EXPR to eval BLOCK at all. It's a completely different operator. In Java, C++ and Perl6, it's known as try. It won't help you at all here.