Easiest just to change your export to test whether to set PERL5OPT:

    if `perl -MInternal::Module -e0 2>/dev/null`; then export PERL5OPT
+=-MInternal::Module; fi
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Though possibly you want PERL5OPT="-MInternal::Module $PERL5OPT" in case the env var is being used for something else too?