My guess:

The OP was highlighting a modperl environment, which is notorious for global side effects IIRC.

So if one of the other modperl clients is manipulating the error mechanism, (like setting $SIG{WARN} or CGI::Carp or something homebrewn), this could lead to some slowdown.(Disclaimer: I never tried this, especially never tried modperl)

It's hard to tell why this is triggered by eval {BLOCK} , but this could be connected to ignoring that special variable which indicates the handler was fired inside an eval. (See $^S in perlvar )

All of this is of course not related to JSON ... and switching to another technology like fastcgi or PSGI should fix it.

Tho I wouldn't be surprised if the OP fiddled with error handlers by himself without taking care of $^S