The gotcha here is that the opening token @' or @" has to be at the end of the line, and the closing token, '@ or "@, has to be a the beginning of a line.

Wrong:

@'1
2
3
'@

Unrecognized token in source text.
At ...ps1:1 char:1
+  <<<< @'1
    + CategoryInfo          : ParserError: (:) [], ParseException
    + FullyQualifiedErrorId : UnrecognizedToken

@'
1
2
3'@

The string starting:
At ...:12 char:6
+ $s =  <<<< @'
is missing the terminator: '@.
At ...ps1:34 char:3
+ #> <<<<
    + CategoryInfo          : ParserError: (1
2
3'@
:String) [], ParseException
    + FullyQualifiedErrorId : TerminatorExpectedAtEndOfString

Correct:

@'
1
2
3
'@

1
2
3