my $p = "\"\$sym{'\$b'}/helloworld/test.dat\"";
# my $p = 'qq[$sym{'$b'}/helloworld/test.dat]';  # alternative
# my $p = '"$sym{\'$b\'}/helloworld/test.dat"';  # alternative
# my $p = q["$sym{'$b'}/helloworld/test.dat"];   # alternative
$sym{'$b'} = "my";
my $val = eval $p;
print "p = $val\n";