use happens at compile time, so any code in external modules actually runs in syntax check mode.

Taint mode "behaves", as you can easily find out yourself

$ perl -Tce 'BEGIN { system <> }'
echo foo
Insecure $ENV{PATH} while running with -T switch at -e line 1, <> line
+ 1.
BEGIN failed--compilation aborted at -e line 1, <> line 1.
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perl -c script.pl is not safer than running perl script.pl if script.pl is malicious. Just the chances of accidental "bad" code are smaller.

(Update: a few wording improvements)

Second update: constant folding also happens at compile time, but that's hardly unsafe.