You have to put /x on the expression when you declare it. Also, as said, escape the literal spaces in the tags, or use \s if you don't mind matching any whitespace.

my $pattern = qr{
    <PgfTag\ `Normal\ \(T\)'> 
    \s* \n
    \s* 
    <ParaLine 
    \s* \n
    \s*
    <String\ `$oldLabel'> 
    \s* \n
   }x;
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