Some websites do different things for different clients. I don't know what agent mech-dump is using, but it lets you specify it, so try telling it the same agent as the one in your script, like:

$ mech-dump --agent-alias='Windows Mozilla' url
[download]

Note: I don't work on HTML/HTTP applications...

Update: I notice that you put single quotes inside your $m->get() call. You're not putting a variable in single quotes there, did you?

...roboticus

When your only tool is a hammer, all problems look like your thumb.