my $pattern = qr{<PgfTag `Normal \(T\)'>\s*\n\s*<ParaLine\s*\n\s*<String `$oldLabel'>\s*\n};
my $replacement = "<PgfTag `Block Label \(T\)'>\n<ParaLine\n<String `$newLabel'>\n";
my $k = 0;
$$contents =~ s/$pattern/++$k && $replacement/ge;