Install Time::Limit and then run:

prove -MTime::Limit=60 -Ilib /path/to/tests
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(This works less well if you are using forkprove, but should be fine with the standard prove tool.)

Or in an individual test:

use Time::Limit "30";   # quote marks around the number are required!
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Update: actually it works OK with forkprove, if you get Time::Limit to send its kill signal to the process group:

forkprove -MTime::Limit=-group,60 -Ilib /path/to/tests
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