If you can use images for the creation of the text, then maybe you could use Image::Magick to draw the text and also find the dimensions of the text.

$image->Read('How long am I');
($text_width) = $image->Get('width');
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I found this information in "Programming Web Graphics" by Shawn P. Wallace (pg. 141) This is the only way I know of to find the pixel width of an object on a webpage.
Here is a quick script so you can try it out and see if it's what your looking for.
HTH

use Image::Magick;

my $image = Image::Magick->new();

my $ps = 15;
$image->Set(pointsize=>$ps);
$image->Read("Label: Test ");
my $l = $image->Get('width');
print $l, "\n";
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