$ perl -Mwarnings -e '$a=inf; print ++$a,"\n"'
Unquoted string "inf" may clash with future reserved word at -e line 1.
ing

$ perl -Mstrict -e '$a=inf; print ++$a,"\n"'
Bareword "inf" not allowed while "strict subs" in use at -e line 1.
Execution of -e aborted due to compilation errors.