For starters, my/helloworld/test.dat is not valid Perl. (bare word my divided by bare word helloworld divided by bare word test concatenated with bare word dat?) So add quotes.

Two, you want the $vars to be in $p instead of being interpolated, so use single quotes or escape the $ (and any @) in the literal.

  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";
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