If convert is on your system, chances are so is Image::Magick.

Not in my experience; usually the Perl module has to be installed separately. How easy that is depends on what Perl you're using, for example, one could use the system Perl and just install libimage-magick-perl, but building ImageMagick (or GraphicsMagick nowadays, apparently) and its Perl module can be a can of worms.

Is there a reason to not use a pure Perl implementation rather than forking a convert process?

Simplicity, mostly. This was only intended for *NIX (for now), I wasn't concerned about performance, and since I'm using my own IPC::Run3::Shell module, I can be sure to avoid all the pitfalls of running external commands. Using the Perl module would be better if I was doing multiple operations on the image that are hard to do in a single command line, but the convert command line I'm using is simple enough.

>usually the Perl module has to be installed separately

Sorry, I meant that the Perl module comes with installing ImageMagick itself if you do it from source. From my experience installing it from source is straightforward (they make it so necessarily). Just a thought I wanted to mention since I've use gone down a similar route with image manipulation. I didn't do any shrinking, but added some watermarks to images I was serving. I also served up different "sizes". But it was all done via the Perl library, not convert itself. Anyway, I think your use of Perl is really neat here.

"Sorry, I meant that the Perl module comes with installing ImageMagick itself if you do it from source."

This is not true. As documented a default install does not build Image::Magick. You can see problems just by looking at the test matrix and open bugs. See also Cannot compile Image::Magick.